When using Indefinite Pronouns, the verb is in the "3rd personal singular" form. When adding a Tag Question, we invert the sense of the verb (+ to - and - to +). So, it's OK to say:
Somebody is there, isn't it?
Nobody has arrived, have they?
I'm sure about the first tag question, but not about the second one. Is the tag OK? If I write "hasn't they" it doesn't make any sense to me.
I'll appreciate if somebody could help me solving this doubt.