You can add a client to an existing group two different ways.
Go to your client index page and check the box next to the client(s) you want to add to a group. Use the action drop-down on the right and select Add To Existing Group.
Use the drop-down and select the correct group you'd like to add the client to. Once selected, click Update Group and you're all set!
Go to the Group tab in the sidebar menu and find the group you'd like to add to.
Click on the EDIT link in that group's row.
Start typing the clients name in the Clients field and select the correct client name to add.
You can add as many clients as you'd like. Once you've added everyone, click Update Group and you're all set!