There are four other plurals in Sindarin - being able to say 'all of something' or 'a group of something'. They are all suffixes, which means they attach to the end of the word.

-ath

This plural ending makes the word mean 'all of the x' and can be used on 'things' as well as peoples.
If the word ends in any letter that isn't mentioned in the table below, then you can just add -ath onto it without changing it.

If the last letter(s) is/are	Then change it to (before adding the suffix)
A	O - This becomes OTH not OATH
F	V
M/MP	MM
N/NN	NN
NC	NG
NT	NN
OL	L
S	SS
U	W

Examples:
Feloth = All caves (Fela = Cave)
Halvath = All seashells (Half = Seashell)
Cammath = All hands (Cam = Hand)
Hammath = All garments (Hamp = Garment)
Aerlinnath = All holy songs (Aerlinn = Holy song, song about the sea)
Angath = All jaws (Anc = Jaw)
Pennath = All tales (Pent = Tale)
Glandaglath = All boundary markers (Glandagol = Boundary marker)
Celwath = All springs (Celu = Spring)

-rim

This suffix means 'host/a group of/a tribe of' and is used on nouns.
However you cannot simply attach -rim onto the end of any noun, it has its own mini set of rules to follow.
Any word that ends in a letter not mentioned in this table below can simply have -rim added to it (including R, cf. Rochirrim).

If the last letter(s) of the word are	Then the suffix becomes
L	-lim
N/NN (derived from an older ND ending)	-ndrim
N	-dhrim
NT	-ndrim
S/SS/ST	-thrim

Examples:
Onodrim = A tribe of Ents (Onod = Ent)
Edhellim = A group of Elves (Edhel = Elf)
Rododhrim = A group of Ainur (Rodon = Ainu)
Dethrim = A group of young women (Dess = Young woman)

How do I know if a word has an older ND ending?
The only way to know this is to check the dictionary to see if it has an older Noldorin version in brackets after it, like this:
talagan (N. talagand) n. harper
Because the older version of the word ended in ND, it means that when -rim is added to this word it becomes Talagandrim = A tribe of harpers.

-hoth

This suffix means 'host', and is usually used for evil or unfriendly peoples or creatures. It has no rule to follow and can just be added onto the end of a word.

Example:
Glamhoth = 'noisy host' (Orcs)
Lossoth = Snowmen (of Forochel)

-waith

This suffix simply means 'people' or 'peoples', but can only be used for Free Peoples (Elves, Men, Dwarves etc. - no evil peoples, Orcs and the like). It also has no rule to follow, you can just add the suffix to any word.

Example:
Forodwaith = North peoples, (Northmen)