chgrp with user

[Xen]

I am trying to get a webserver to run under my regular user, or at least 
to have the website's files under control of my regular user, but the 
webserver runs as www-data.

But it seems I cannot do chgrp as a user.
Is there any way to achieve this?

I mean I could add my user's group to the www-data user and I guess then 
I'd have access but I don't really see how changing a file's group could 
be a security risk. Maybe I should add myself to www-data instead?