There are two ways to do this. The first and the one I would recommend considering it sounds like you will allow more than one or two people to perform this function is to create a security group say AddComputers. In Add Computers and Users snap-in, right click the domain and select properties, Select security, select advanced, select the group that you just created, select view/edit and check the following:
Read Account Restriction
Write Account Restriction
Reset Password
Validate Write to DNS Host Name
Validate Write to Service Principle Name
These are the minumum ACL's to allow someone to join a server or computer to a domain.
The other way is to individually check these ACL's for each person you want to allow this function. Good luck.
Jeffery Smith (Smitty)
PEC Solutions Inc.
BS - Computer Application & Networking
A+ Network+ MCSA MCSE