Looks like there isn't a way. But the next best thing for me is to let people press 'play now' and then hit them with a message box that says 'No' - you have to invite people and that looks like this.
- (void)matchmakerViewController:(GKMatchmakerViewController *)viewController didFindMatch:(GKMatch *)match
{
if(_matchRequest.playersToInvite == nil)
{
//This happens when the end-user presses 'play now'
//Don't let them 'play now' - force them to invite one other device
UIAlertView *v = [[UIAlertView alloc] initWithTitle:@"No" message:@"You can't play now. You have to invite someone." delegate:self cancelButtonTitle:@"Ok" otherButtonTitles:nil];
[v show];
}
else
{
//you invited someone - you can start the game
_match = match;
//...
}