To invite your friends to join a group, tap on the group in the group list tab, and then go to the "Members" tab. Tap "Invite friends."
If you want to invite friends from your phone contacts, allow the app to access your contacts (they won't be uploaded or stored) and select the friend's name in the list. Tap "Done" when you've selected all the friends you want to invite. You can also tap "Done" right away and enter a number or email manually. You'll have a chance to add your own message before the invitation is sent; invitations will be sent using your phone's messaging plan or iMessage, or your own email account.
When your friends tap on the invitation link, they'll see a special welcome message and will be able to join your group right after they create an account. You'll get a notification when they join if you enable notifications or emails for new group members.