My back up idea is a bulletin board in the tower. It could be like a mini-forum at the post office. Steps: Post office -> bulletin board -> category, e.g. Raids -> select "Create Fireteam" or request to join a player who has already done it. If another player requests to join you, you can accept, or decline via a notification like the ones we already see for invites. Canceling your post, or leaving your team could be the same as normally leaving a fireteam. You could also have categories for strikes, crucible, clans, and other social functions.