In User Accounts (in WGM), go to the Group TAB and click the "+". This will show all your groups in the slide out drawer that will appear on the right side. If you have an OD Master, you will need to switch from the local Groups, to the Open Directory Groups. Can describe it real well, but it will be above the list in the slide out drawer. You will want to be in /LDAPv3/127.0.0.1. You will then see all the available Groups.
BUT, why would you want to add anyone to these groups? It would be better to create your own groups for management purposes.
Just so that if I have to add myself to admin again this year (I think our server went down again big time) I can. I don't want to have to ask the previous admin how to. My OS X Help Desk Essentials book should be here Friday.