Go to Tribes->My Tribes in the menu, and choose the Tribe you'd like to invite people to. Within the Tribe, choose the Members item.
You can invite Racefully users to your Tribe by choosing them from your Friends list, or looking them up in the app. You can also invite people outside of Racefully using a special link that will take them straight to your Tribe when they open it on their phones.
The Members area also has options to remove members from the Tribe, and to cancel invitations. Use the Edit pencil in the top-right of the screen, then select the members or invitations you wish to delete. Press the Stop invites button to remove the selected Tribe members.