Groups provide a convenient way to grant and revoke permissions to more than one user at once. Every user is a member of the “public” group and can be a member of one other group.
Tasks involving groups are:
Creating and deleting groups
Adding users to and deleting users from a group
Granting and revoking object permissions
Granting and revoking command permissions
It may be convenient to create groups before adding users to a database so you can assign new users to existing groups.