How do I set how a group's new expenses are split?

Open the group, tap the gear for Group settings, then New expenses split. Choose Equally among the people you pick, Shares or Percentages, and tap Save, the checkmark at the top.

Every new expense in the group then starts split that way, on everyone's phone, and its Split row names the group's default. Each expense can still be split another way, and editing an expense never changes its split.

Anyone in the group can change it, and it costs nothing. Picking everyone under Equally removes the default, since that is how a new expense starts anyway.

Someone who joins later is not in it until someone adds them. Someone who leaves is dropped from it; if they held a percentage, the group says the default no longer fits, and new expenses split equally among everyone until it is changed.