you have 4 friends in a chat party and 1 person finds a free for all and invites the rest of the party really quick, then the person who invited and found the free for all backs out and gets re-invited by one of the 3 others in the lobby once this person rejoins the lobby is locked.
You can then invite more people if u want and it will still be locked.
Also people cant join session they have to be invited to be able to join, so you don't have to appear offline.

get 4 or more people in party chat and then find a free for all invite you 3 friends straight away then back out once they have all joined then get 1 of them to invite you back. if there is a random in your game you need to do it again and if not its locked. even you friends cant join session with you the can only be invited in.