Some region commands require you to stand inside the region where you want to apply the command.
So may the town commands too. Did you make sure of that?
I'm not in a town so I can't help you figuring.
Make sure to write the region's/town's name exactly as it is, including capital letters, symbols, etc.
Maybe the command must be:
/town region add guest Aincrad <playername>
As in /town region add <guest|manager|owner> <region> <player>
Also, just to clarify something, you ARE talking about a town's region? Not the town itself? There's a difference between the town and town regions - or subzones.
To check if the region exists do this command:
/town region info <regionName>
Try all that while standing inside the town's region. If it doesn't work, a variable is wrong, the region name is wrong or the region doesn't exist, or you don't have permissions.
See here again all commands:
http://herocraftonline.com/main/wiki/list-of-commands/