To join a group, you must be invited by a current member of the group. If you receive an invitation into a group, you'll see a little speech bubble next to the group that you have currently open. Tap on it to open group selection, then tap "Join group" to accept the invitation and join the group. If you haven't downloaded FAMILO yet, you'll get a SMS with a clickable link. This takes you to FAMILO in the App Store/Google Play Store. After opening the app and logging in with your mobile phone number, you can accept the invitation and join the group.