Anyone can "claim" land, if you build a house on Bastion and put sign with your name on it, it is yours and cannot be griefed.
However, no, guilds do not get regions. Often Guilds are based out of a town's region, or out of one or more supporter residence region.
The reason guilds do not get regions is because anyone can make a guild with little cost to themselves, and the map would be overrun with guild regions.
Edit: I have "claimed" small surface areas with fences (my residence's lawn, for example) but Danda is correct in that people still dig around underneath my lawn like little gophers trying to AoE me, and its all legal because I had not utilized the space under my lawn yet.
Had I included some underground "LOL wall of cauldrons, pistons, and enchanting tables", I doubt anyone would try to get in because of the potential annoying fix for their grief would take three separate professions. The "LOL wall of cauldrons, pistons, and enchanting tables" is difficult to set up, but once placed (hidden BEHIND ur aesthetic building materials of course), it is relatively impenetrable on any raider's first raid attempt on your base.
This could be a useful wall for a newish guild to try using if they have access to multiple professions that have monopolies on certain block placements.
Edit: I agree with dandas posts, and it is impossible to entirely keep people out of an unofficially "claimed" area.