Once someone is deemed worthy of joining the core team, you (a core member) should make a proposal on the core mailing list to welcome the new developer. Subject should clearly state the proposal and give ample time for other developers to chime in. Usually all core team members should give their explicit approval as we try to reach consensus (and usually do) over proposals.
Once the member is approved on the mailing list, you should follow this checklist:
Give membership on the mailing list.
Grant commit access to the core repositories (Hostmaster, Hosting, Provision and Eldir).