I tried to install mariadb55-server with user:group facs:wheel instead of the default mysql:mysql.  But the build failed because it couldn't find wheel in a file in /usr/ports:

===>  Staging for mariadb55-server-5.5.39
===>   Generating temporary packing list
===> Creating users and/or groups.
** Cannot find any information about group `wheel' in /usr/ports/GIDs.
*** [create-users-groups] Error code 1

That doesn't seem like a good place to look.  Even if it finds something, there's no guarantee that there won't be a conflict with the local value of whatever identifier it finds.  Unless there's something I'm missing about this, I'd think it should be looking for an existing local declaration instead (/etc/group?)