How does multiplayer work?

Hello hunters, how does multiplayer work? Ive never been able to join another session, but ive had players join mine, yet they never respond to me, or try to play together.

I have been hoping to come across someone whos willing to help me with quests, especially the dreaded Nergigante. But nobody even seems to recognize that theyve joined another persons session.

Ive even tried to directly message the person by singling them out in the chat settings, but i get no response. Is there like a friend system that you need to use to chat? Am i missing something?