Heyo! 
This happens every time you make a party. When your in a party by yourself and someone invites you to his/her party, you have to disband the party. When you disband it, it kicks you out so you have to do /p (or /party) to open up your party menu again. They should make it so it brings you back to your “Party Invite tab”. This will save a lot of time, and especially when your in a game, you want to join your friends party as fast as you can before you get killed.
(Sorry if explanation is bad, just let me know if you don’t understand what I mean.)