Parties are groups that can communicate and easily move across different app experiences together. Users in parties will have a System VoIP connection allowing them to talk to everyone in their party. Users can only be in a single party at a time, but they are not necessarily always in a party. Parties are primarily intended to be a system that allows users to more easily communicate and organize meetings with their friends in VR.
It's possible to fetch the list of users in a room with ovr_Party_GetUsers.
It's possible to fetch a list of all currently invited users with ovr_Party_GetInvitedUsers.