I have had this problem since beta/early access.
I get a 4-5 second client freeze (screen freezes, no control of character) whenever someone is added to my group, removed from my group, or when someone in the group disconnects or reconnects to the game.
If the group size is 13 or more, this also crashes my game client, and I cannot log back into the game at all until the group leader kicks me from group, or until the group drops back to 12 or less people.
Anyone else having this issue?