My client has a domain pointed to a group platform now

My client has a domain pointed to a group platform now and we need to build a site for it.
Is there a way to point the domain to my server and set up multisite as usual, then add .htaccess to redirect users while the site is being build on the server.

Basically add a rule to redirect all users to the group site and add exceptions to the rule to exclude the ip's of us who are working on it.

Seems I have done it before, but my ram is full and I just can't pull the "how to" forward.

Thanks for your time.
Richard