Go to the Setting wheel image>Group settings>Members. Now you can decide whether you want to invite members by E-Mail to join the group, or whether you want to add the members.
If you have chosen the first way, go to Invite members, write an invitation and click Send invitations.
If you have chosen the second way, go to Direct add members, write to the E-Mail addresses of the user and may be a welcome message in the text boxes and click Add.
Nico