Send them free gifts once per day. Challenge them, and share invitations to the special course when you unlock them. Special stage invitations are automatically send to your friends.
Select the Community button from the Main Menu to see your friends list. In the friend list you can see if friends are online and see your standing friendly match total wins and losses versus that friend. Select "Send to all" to send free club boxes to everyone on your friends list. Select a friend to challenge them to a match, or if a friend is in a Match, an eye icon will appear near their name, indicating the Spectate option is available.