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 sysutils/symon for an example. Your port must use a fixed user/group ID number. 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).
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.