Is there a way to make it so friends can join other friends who are in reserved servers?

Been working on a sort of single player game however I want people to be able to play with friends. Now my first thought of how to do that is to create reserved severs which I have done however I was wondering if there is a way for friends to be able to join friends who are already in a reserved server when they are invited or something.

Any other ideas of how to do this would also be very helpful thank :)