Somewhere on the Coliseum button (maybe where it shows the WAITING animation), show how many players are currently waiting or are in-game to encourage more players to join in.
A similar thing has already been done with the Party Finder button, where it shows Party Found - 5 (for example) but you're required to first start finding parties to see that.

Ideally both of these counts should be shown before you having to click on the Party Finder or Coliseum buttons, to encourage more players to join.