To add members to your Howbout group:

1. Head to the People page by tapping on the people icon in the bottom right of the screen
2. Tap on the group you want to add members to 
3. Once viewing the group, tap on the group options icon in the top right of the screen 
4. Then you can select 'Add friends' to add members who you are already friends with on Howbout, or tap 'Invite link' to invite them via whatever chat app you like

Adding new group members will automatically invite them to all upcoming group events.