In the create (and edit) trip pages, include a "auto-approve" number field. That number should default to zero and be zero minimum.

The auto-approve field in the database should be:

auto_approved_members INTEGER NOT NULL DEFAULT 0

On the server side, members being added or removed from a trip should trigger a function that checks whether the trip is under its auto_approved_members limit and then auto-admits more accordingly; this function should never automatically remove users from the trip. The users are that are admitted should receive emails.

This function should only operate if the trip start time is greater than 24 hours from now. Make sure that information is communicated on the Create Trip page.