Free Roam events are random pvp events that only trigger while you are in free roam mode. You have no control over them and they spawn randomly. From the ones I've played I had one spawn at 15 mins to the hour and another one at the top of the hour so it could be every 45 mins or just those 2 intervals. I will update if I see any trends.
When available, you will see a purple notification on the top left corner of your screen. Hold the menu button then A to accept the invite. The game mode will be anything pvp from Master Archery to King of the Castle you have to compete in. If you miss the invite just press left on the d-pad and go to invites to accept. You have a limited time before the invite expires You just have to complete the event to get credit but you will also receive XP and Money based on your performance.