When adding a new party member, the potential member rolls a d8. The resulting number on the die must be higher than the current number of party members. This symbolizes the process in the world that the gods have placed restrictions on parties to keep them from overwhelming others.
If the dice roll fails, the potential member must wait a day (in the world) before being able to join a party.
A party member may leave the party of their own volition. Any avatar only has the chance to join a party once per day (in the world).
A non-party member may tag-along with a party, but they do not have access to party abilities.
As an added bounce: The GM may add the number of sessions that the player has participated to the roll. Never to exceed 8 members in a party.