That's as easy as it can be. All we do is type
groupadd htmladmins
and we have group for all of those html administrators (to see all groups look into /etc/group). Of course we have to add user to this group, so we edit the /etc/group file, look for the line with our new group and simply append testuser to it. The line should then look something like this:
htmladmins:x:501:testuser
The only thing that might look different on your installation might be the running number 501. If you don't specify a GID, this number is always increased by one.