Guesting lets you play with people in other servers, so if you managed to get a party but none of your party members are in overflow shards, you just guest to a server. There's still that 2 world per day restriction though, but I imagine it's better than nothing.
Connor, when you see a server FULL means that it is full from people actually playing there at the moment, independently of where did they create their accounts. For example, if a player creates an account in Vizunah Square and then moves to Baruch Bay, then all the characters of this player will go to Baruch Bay and he will be counted among the total amount of players of Baruch.
Im assuming it probably has something to do with the new map coming out later this month
No, tha'ts not it. The new map coming this month is an island and it'll be accesible from an Asura gate in LA.
I've seen that before too, methinks it's just there for future content updates or expansions. There other places where it looks like it will take you to another zone but is just blocked - like in Sparklfy Fen and Caledon Forest.