Organizers can start a group and send out invitations by email or text message. Without an invitation, you cannot look up a group. Joinable groups are private by default settings. If you have been invited to a group, you can create a new Joinable account, and if your email and/or phone number match, you'll see the group listed in your active groups (click the menu link in the top/left part of the app). If you still don't see the group, let the organizer know which email you're using and have them re-send the invitation.
You can be invited to multiple groups and will be able to see them all listed in the global group navigation menu located on the left hand side of your screen. To pivot between them, just click on the group you would like to view.