Yes, players who joined your event will get an automated reminder message, 24 hours before the event starts. This will be a push notification for players who have the app, and an email for players who have an online account only (and/or don’t see their app notification).
Note: If somebody joins an event less than 24 hours before it begins, they will not receive a reminder notification.
However, you can not set up/customize event reminder notifications.
The first screenshot below shows an OpenSports push notification coming through the phone while the player is in another app, and second screenshot shows the notification in OpenSports.