Go to groups.google.com and click the Add Members button. You can then specify the level of membership, a welcome message(typically used to notify a user that they have been added and what the group is about) and their subscription level (seen right).
If you need to remove a member just click the group name to go directly into the group , then click Members on the left. Select the user you want to remove and then click the - (minus) in the top right. From this page you can also mass remove members or edit their subscription settings (seen below).