Power Users don't exist in Server 2008 at all, not just SBS.
The ability to create users is actually a *big* privilege as it means direct access to AD. There is no good easy way to grant that acces in a small AD environment without handing over the entire "keys to the castle."
You'll basically need to create a new security group, grant it privileges via security policies, and then use delegated permissoins via the AD snap-ins directly. Then make the user a member of that group.
It won't be easy, it won't be fun to maintain, and it'll be an auditing nightmare. In short, only do this if absolutely necessary. Spell out the time and cost investment and make sure that they actually want to do this.
-Cliff