Go to your group's main page, then click Manage group members. On the Manage group members page, select check boxes for the members you want to manage.
For the members you've selected, you can ban them from participating, delete their membership, send them messages, and more. You can also export their member information as a contact list you can then import into another application, such as an email address book.
You can change a selected member's role from administrator to member, or back the other way. An administrator role lets the member make the same kinds of changes you can make as the group's owner (except delete the group).