How do I add a new person to a group?
On iPhone: First, navigate to your group, then tap the gear icon in the upper-right corner of the screen to go to the "Group settings" screen. On that screen, you will see a list of all group members, with a button at the bottom of the list to add more members.
On Android: First, navigate to your group, then tap the menu button in the upper-right corner of the screen and choose "Group settings". On the "Edit group settings" screen, you will see a list of all group members, with a button at the top of the list to add more members.
Also, note that you cannot add yourself to an existing group – one of your friends who is already in the group will need to invite you.