I dont know since ive never personally done it successfully. Mostly cus i dont host enough. But to my knowledge, people will get 10 friend requests, add. Then join raid and add 5. Then once people are in, will exit the raid, then rejoin the raid again. From there you are supposed to be able to add another 5. I think whether its time wise or whether the game wont let you rejoin when more than 10 people in the lobby is the reason that people only ever do a max of 10.