If they have admin privileges. The person who creates a group has them by default, and can grant them to anyone else, admins can add, remove, and grant or revoke admin privileges and set group name/description parameters, and disappearing message configuration. Yes, you could have a group where the founder revokes admin privileges for themselves and then nobody can make changes to the group (although individual members can leave and delete the history on their own devices). Signal users can also delete their own messages.