To leave a Howbout group:

1. Head to the People (👤) tab
2. Choose the group you want to leave 
3. Once viewing the group, tap on the group options icon in the top right of the screen 
4. Tap 'Leave group', then hit 'Leave' in the prompt that appears to confirm

Leaving the group will remove you from all group events. If the group creator leaves the group, the first person to have joined the group after its creation will become the group admin.