The current list of reserved UIDs and GIDs can be found in ports/UIDs and ports/GIDs.
If your port requires a certain user to be on the installed system, let the pkg-install script call pw to create it automatically. Look at net/cvsup-mirror for an example. Please note that this is strongly discouraged, please register user/group ID numbers as stated below.
If your port must use the same user/group ID number when it is installed as a binary package as when it was compiled, then you must choose a free UID from 50 to 999 and register it either in ports/UIDs (for users) or in ports/GIDs (for groups). Look at japanese/Wnn6 for an example.
Make sure you do not use a UID already used by the system or other ports.
Please include a patch against these two files when you require a new user or group to be created for your port.