From the main calendar screen in OurCal:

1. In the top left, tap the OurCal logo
2. Tap “Invite Members”
3. Share your group's invite link

Newly invited members will need to follow your invite link to download OurCal, where they'll be prompted to select "Join an Exiting Group".

Once joined they will be a member of your group with access to all shared calendars, their own member calendar, and a private calendar.

They'll also have the ability to create additional calendars for the group, add events, and send messages.