The crowd bar is the little bar at the bottom of the character HUD that goes up to 500.
To fill it, you will need to complete small tasks as they're set throughout the match. these can be things like perform a submission move (hold
when next to a downed opponent), hit an opponent following an Irish whip (
), hit a punch combo (
) etc. Complete a few of these and the bar will fill before you know it.
Don't stand around for too long without doing anything, though, as inactivity will lower the crowd bar value significantly, setting your progress back a lot.