Singularia tantum. This term refers to nouns which only have a singular form, whose plural status is defined by surrounding words and context. Typically these are only mass nouns used for "uncountable" things like dust, milk, etc. If multiple instances of the thing these nouns refer to exist that is represented by surrounding words: jugs of milk, piles of dust, etc. There are, however, two examples of singularia tantum words which are not mass nouns, both of which are pronouns. The first being the word 'they,' and the second one (the one most powerful in an argument on this topic) being the word 'you.'

They pick up a book. They all pick up books.

You must leave the room. You must all leave the room.

This is notably different from invariable nouns, which refers to a noun whose form never changes at all like sheep, deer, moose, etc. This is because 'they' and 'you' do change in other ways just like other singularia tantum words, just not in a way that gives them a distinct plural form.

That is a milky white color. I milked the cow.

This place is dusty. I need to do some dusting.

They're going to the store. They've done it.

You're awesome. You'll be missed.

So next time someone says, "So are you two people now?" or "Is there more then one of you?" Bust out the term 'singularia tantum' and show them just how stupid of a hill to die on the gramatically plural argument really is.

(Bonus bit. The opposite of singularia tantum is pluralia tantum. Words that only have a plural form like scissors and pants. Singular instances of these are determined by surrounding words as well. A pair of scissors. A pair of pants. So even if someone tries to argue that they/them is never singular there is an answer for that too. It's either singularia tantum or it's pluralia tantum. Either way the point remains, the singular and plural forms are identical and their singularity or plurality are determined by surrounding words and context.)