Now I believe I understand how the campaign/online works. Its all sort of meshed into one. But I would like to know, Is it possible to do something with friends outside of the campaign/online around the ark. What Im trying to say, is it sort of like a sandbox where me and my friends could have a race across the ark if we wanted to?

nope normal multiplayer style levels with story style cutscenes

This is true, but i know for a fact you can set all enemys to Bots, but i have heard rumours, you can turn bots off.
So it would be just like playing in a freemode, or sandbox gametype. But this is all just speculation for now. Hopefully its official!

it's still going to be singular regions though not run from container city to the reactor the fastest. the levels might be fairly large though so i wouldn't get discouraged just yet.

Don′t forget the Challenge mode. In this mode you can unlock new weapon upgrades in singleplayer and coop.
Have a look at this -> http://i251.photobucket.com/albums/gg282/xH0Rsix/Brink/BRINK_challenges.png

I dont′ know what "Freeplay" means, but this could be your wished "sandbox" mode. Not for sure!

There is no sandbox-element in BRINK. http://www.gamesas.com/index.php?/topic/1097007-any-free-roam/ is pretty much the exact same thread, btw.