Happens regularly to me on PS4 - start a chat party and the list reappears (don't need to invite anyone to it)

Hello Nippelia, There are a couple things that some users reported as temporarily helping, like shutting off your console, clearing your cache, or just using the console's friends list to join your game. However, if none of these work, you're not alone, there is a bug that's making it so some user's in-game rosters aren't populating. Bungie is aware of the issue and they are investigating. https://www.bungie.net/Mobi/en/About/Article?id=44670

It happened to me before, my whole friends list disappeared from my roster. After deleting all pending friend requests and restarting the game it was fixed.