Sindarin has a second plural form using suffixes like -ath, -hoth or -rim. Compare:

• êl “star”, elin “stars”, elenath “all stars”.
• orch “orc”, yrch “orcs”, orchoth “all orcs”.
• Nogoth “Dwarf”, Negyth “Dwarves”, Nogothrim “all Dwarves”.

Tolkien himself usually called this the “collective” or “group” plural, but in the literature on (Neo) Sindarin this second plural is usually referred to as the “class plural”, and I follow that convention to avoid confusion. The notion of a class or collective plural dates all the way back to Gnomish of the 1910s, and Tolkien mentioned this feature of the language frequently throughout his life.

... the Eldarin languages show a tendency also in the plural to make a distinction between the “partitive plural” — men, some men; and the group plural — the men, all the men of a group or kind, the whole body ... In Exilic Noldorin the partitive form was lost and the old general plural extended its use; but new group names of peoples were made with suffixed hoth, rim etc. — Golodh, Gnome, Gelydh, Gnomes, Golodhrim, the Gnomes (Primitive Quendian Structure: Final Consonants, 1936, PE21/57 and note #24).

The Eldarin languages distinguish in forms and use between a “partitive” or “particular” plural, and the general or total plural. Thus yrch “orcs, some orcs, des orques” occurs in vol I pp. 359, 402; the Orcs, as a race, or the whole of a group previously mentioned would have been orchoth. In Grey-elven the general plurals were very frequently made by adding to a name (or a place-name) some word meaning “tribe, host, horde, people”. So Haradrim the Southrons: Q. rimbe, S. rim, host; Onod-rim the Ents (1954 letter to Naomi Mitchison, Let/178).

Both Quenya & Sindarin have for most nouns two plural formations: the general or group, and the partitive or special. The plural element in nouns is [i] as a suffix [ī] ... In Sindarin the old ī plurals causing “affection” are partial (as a rule) and the group plurals have -ath: a group suffix, or other endings as -rim (draft of a 1955 letter to David Masson, PE17/62).

... the suffix -ath (originally a collective noun suffix) was used as a group plural, embracing all things of the same name or those associated in some special arrangment or organization. So elenath means “the host of stars”, sc. (all) the (visible) stars of the firmament. Cf. ennorath, the group of central lands, making up Middle-earth. Note also Argonath “the pair of royal stones” at the entrance to Gondor; Periannath, “the Hobbits (as a race)” as collective pl. of perian, “halfing” (pl. periain) (The Road Goes Ever On, 1967, RGEO/66).

ath: Though it cd. be an S. form of Q. atta “2”, it is not in fact related, nor a sign of a dual. It was a collective or group suffix, and the nouns so formed [were] originally singulars. But they were later treated as pl[ural]s, especially when applied to people(s) ... e.g. Periannath the Hobbit-folk, as distinguished from periain hobbits, an indefinite number of “halflings” (1972 letter to Richard Jeffery, Let/427).

Probably the simplest English translation of the class plural would be to put “all” before plural form. This could be either in the sense “all of the thing in existence” or “all of the thing in this particular context”. As Tolkien said above, orchoth could mean “the Orcs, as a race, or the whole of a group previously mentioned” (Let/178). Most of the attested examples of the class plural seem to use it in the sense of “all in existence”, but in the King’s Letter the class plural ionnath was used to refer to the group of all the sons of Samwise:

• ar Iorhael, Gelir, Cordof, ar Baravorn, ionnath dîn “and Frodo, Merry, Pippin and Hamfast his sons [ionn-class-plural]” (SD/129).

Perhaps the most extreme example of the class plural being used with a specific group is the Argonath “(pair of) Royal Stones” (RGEO/67), where the “group” consists of only two items, though at other points Tolkien did consider making this a dual formation (RC/347).

Some class plural suffixes -rim, -hoth and -waith are used for groups of people: orchoth, Rohirrim, Forodwaith. Of these, -hoth is mostly used for “bad” groups given its connotation of “horde”. However, by far the most common class plural suffix is -ath, which seems to be the “default” suffix and the only one used with inanimate items like elenath “all stars” and sammath “(all of the) chambers”. Tolkien connected -ath to ✶atta “two” and related ancient collective suffixes ✶-attā and ✶-astā, the latter also seen in Quenya:

The latter -tta, sta probably did not originally indicate duality, but merely close grouping. Cf. the same suffixes -tta, sta: as in Q tengwesta, collection of writing (tengwe). Also -atta, -asta is a collective suffix, which in {Ilk. >>} Ilk. & N. yields the collective plural ending -ath as in cīr, ship, círiath, ships (Primitive Quendian Structure: Final Consonants, 1936, PE21/57 and note #28).

Only Q. had atta 2. And NB. S. -ath < atta was never limited to dual but was < atta, but asta is used in Q. as group suffix (notes associated with Eldarinwe Leperi are Notessi, late 1960s, VT48/19).

The suffixes -rim, -hoth, -waith mostly formed ordinary compounds, but the suffix -ath could preserve (or restore) ancient final endings to words that were lost in the singular form. Compare:

• êl “star” → elenath “all stars” (LotR/238; PE17/24).
• Feir [sic., expected *Fair] “Mortal” → Firiath “(all) Mortals” (WJ/387).
• gail “star” → giliath “host of stars” (RC/232).
• Perian “Halfling” → Periannath “Hobbits (as a race)” (RGEO/67).
• N. ceir “ship” → círiath “(all) ships” (Ety/KIR; PE21/57).
• N. lhîn “pool” → lhiniath “(all of the) pools” (Ety/KHIS; Ety/LIN¹).
• N. sîr “river” → siriath “(all of the) rivers” (Ety/SIR; RS/433).

In the case of feir, gail, ceir, lhîn, sîr (from primitive *phiryā, gilyā, ciryā, linyē, siri-), the class plural preserved the ancient final y > i and prevented it from intruding into the preceding word. In the case of elen, perian it preserved an n lost in the singular. Furthermore, class plurals can trigger certain diphthong reductions seen in compounds, most notably the rule whereby [au], [ae] became [o], [e] in polysyllables:

• caun “outcry” → conath “outcry of many voices” (PM/362).
• maew “gulls” → mewrim “(all of the) gulls” (WJ/190; Ety/MIW).

Note, however, that as a “recognized compound” the suffix -rim does not induce i-affection in the preceding vowels:

• Aphadon “follower” → Aphadrim (PM/362), not **Ephedrim.
• Onod “Ent” → Onodrim (LotR/1130), not **Enedrim.

There are enough potential variations that the class plural could conceivably be very irregular. We don’t know which historical irregularities would have survived and which would have been regularized by analogy with other forms, or for that matter which forms would be the basis for such regularization.

As for how they might be used in a sentence, Tolkien said “the nouns so formed [were] originally singulars, but they were later treated as pl[ural]s, especially when applied to people(s) (Let/427)”. This means that they use the plural definite article in, which in turn causes nasal mutation as appropriate, for example:

• Dagor-nuin-Giliath “Battle-under-Stars [gail-class-plural]” (S/106).
• Ernil i Pheriannath “Prince of the Halflings [Perian-class-plural]” (LotR/768).

Any adjectives or verbs would likewise be put into the plural to agree with the noun, as in:

• o galadhremmin ennorath “from tree-tangled [galadhremmen-plural] middle-lands [ennor-class-plural]” (LotR/238).
• ar in Ellath epholar eno vi Eressai “*and the Elves [Ell-class-plural] remember [ephola-plural] still in Eressëa” (VT50/5); the exact meaning of this verb is unknown.

Conceptual Development: The concept of a collective plural dates all the way back to the Gnomish Grammar of the 1910s:

Cp. lim many (Q. limbe) ... This is ordinarily suffixed direct to the N[ominative] S[ingular] form, and forms properly a collective plural. The distinction is however often obscure and certain classes of words and special words tend to make more use of -lim suffix than others. This use is common in denoting names of tribes, etc. (GG/15).

At this early stage the main suffix was -lim with variants -thlim and -rim. Other collective plural suffixes like -(h)oth (PE15/61), -waith (PE13/143) and -lhai (PE13/146) gradually appeared over the next couple of decades. Pinpointing the introduction of -ath is tricky. Variant plurals with -ath appear in the Gnomish Lexicon Slips of the late 1910s, as in: alfath vs. ailf plurals of G. alf “swan”; amladath vs. amlaid plurals of G. amlad “surface” (PE13/109). However, these variant plurals might be connected to the Gnomish plural suffix -th rather than being true “class plurals”. By The Etymologies of the 1930s, however, -ath was well established as the main class plural suffix, and remained so thereafter.