As of time of writing, Respawn says that the Titanfall beta is having some issues with the servers, but it is as a result of a bug in the system.
Titanfall developer Jon Shirling tweeted the following:
Titanfall Beta is unable to allocate more servers right now. This is a bug, not a limit. Microsoft is aware and working on it.
If you have been invited to the beta but cannot play the game, be patient until Microsoft fixes this bug or whatever error is preventing more servers to be up.
