How is a Borderlands 2 multiplayer game hosted?

Borderlands 2, like its predecessor, does not use dedicated servers, but relies on the host player (the one whose game the other 3 join) to act as the server. Therefore, you will probably be better off joining other peoples' games together.

Whether you'll be able to comfortably play with a friend depends on your network and geography/Internet topology: somebody from Eurasia will have an acceptable connection to somebody in the Americas (~100-200 ms ping), while somebody in Australia will have an abysmal ping to somebody in Eurasia (~350-450 ms), due to the packets having to go through NA.