Sure, the user  management permissions are separated from the other
permissions. For WSS check out:

Manage site  Groups - Create, change, and delete site groups, including
adding users  to the site groups and specifying which rights are assigned
to a site group.

Manage List Permissions - Grant, deny, or change user permissions to a
list.

Create Cross-Site Groups - Create a group of users that can be granted
access to any site within the site collection.

For Portal:
Manage Area Permissions - Add, remove, or change user rights for an area


You can play with those until you get the rights you desire for your
power users.

Keep in mind that once you have the Manage Groups permission, you can add youself or someone else to the site  Administrators group.

should have looked a little more closely before posting.

The problem with assigning to a user  the “Manage site  Groups” right (and it does not matter what other major rights you enable/disable) is that the user will have the ability to assign himself/herself to the Administrator site group.

I think there is a default site group that is missing from WSS – Site Coordinator. This site group could have its default rights the same as all current Administrator rights except they don’t have the ability to assign anyone to the Administrator site group. Its “Add User” page could have the Administrator checkbox in the “Choose Site Group” section greyed out.