I don't think the guild function is in game yet. But, as far as Pixel Gate City, its on Solum which is a US East server. I am not sure what their requirements are on helping or joining on a grp build is, that is something you'd have to ask the city founder/s. Otherwise, its more of a case of find some place you like place your beacon/plots, and start building. There are many player cities distributed about the other planets, for details on the planets look here. For example, Dragons Watch (also in Solum), there was a city someone was starting that supposedly has a Venice theme to it (i forget the name), and then there is a port town, the currently trending Elop Portas of @Jeffrotheswell in Elopor, and there is the new town I am trying to start on Andooweem Tjentzu (Oceanic Server), and Therka Market in Therka a EU server, as well as I am sure there a a few other planets that presently do not have a city atm.
I think its more about respect to the city founders when you ask permission to build, as opposed to something enforced by game mechanics. But, that is the wonderful thing about this game - people (players) do that.
Note: Do use the search function on the forums to find the cities I have mentioned. Most player cities have a certain theme they are trying to capture.