XP is just the sum total of every players score, so the more you (as a team) score, the more XP you'll recieve!
I'm going to quote myself here:
These show up on the end of match results screen, and are worth
varying amounts of score. Every medal has differing values of Bronze,
Silver and Gold, and the point values scale linearly with weapon kill
medals, and geometrically with everything else. This means that
getting 25 kills with a pistol and 50 kills with an assault rifle nets
the exact same point bonus as getting 75 kills with an assault rifle,
but getting 10 headshots and 1 grab is worth less than getting 20
Some medals are awarded team wide. Mission completion and extraction
bonuses, for instance, as well as going a certain number of waves
without losing a squadmate. Like the individual achievements, these
come in Bronze, Silver, and Gold.
To get your score, the game simply adds all these up.
(A more complete listing is available here.)