You can add existing Community members to a group one of two ways:
1) From the Groups list
2) or from the Group page itself
We'll soon be adding a way to add a brand new member to the group, who is not already in your community. However for now you 'll have to just invite the member using the Invite member button at the top of the app, before adding them to a group.
We'll keep you posted with new developments! :)