Anyone can leave a group anytime. No one is notified when you leave. To leave a group, head to the group tab and then scroll down to “leave”.
Data is handled in a similar fashion to a member removed from the group. Members can rejoin the group at any time and your profile will remain intact. However, joined group chats and event sign-ups will reset. Furthermore, a removed member can continue to use any existing private communications on Heylo.