Joining a child domain

Hi,I’m working on window 2003 server.My company has a big network.I’m enterprise administrator.There are a lot of child domain under the forest.One of my collegue want to join a domain under a child domaing but he couldn’t,it prompts him a message access denied .He asked me to give him the enterprise password but it’s illegall to do so .So what can i do to give him the privelage to join the domain.Thank you.