Learn Icelandic: Hver er sá þarna á samkomunni? - Who is that one over there at the gathering?

In Icelandic, sá is a demonstrative pronoun that means "that" or "that one" when referring to a masculine noun or a person of unknown gender. If you used the neuter það, it would sound like you are asking about an object or an abstract concept, rather than a person.

What if I wanted to point out a female and ask "Who is that one over there?"

You would need to change sá to its feminine nominative form, which is sú. The sentence would then be Hver er sú þarna á samkomunni?

Could I use sá right next to a noun, like "sá maður"?

Yes! While sá stands alone here to mean "that one", it can also be attached to a noun. Saying sá maður (that man) is perfectly correct. In both cases, sá points out someone specific who is further away than "this" (þessi).

Why is the word for "the gathering" (samkomunni) in the dative case here?

The preposition á governs the dative case when it describes a static location, meaning where something is happening right now. Since the person is currently at the gathering (samkoma), it takes the dative definite form samkomunni.