You can play a match with your friend or a fellow clanmate without the need to pay entry fees.
Just bring up your friend list from the Friend menu in the main lobby or select a clanmate from the clan members list.
If the friend is online, you will be able to tap on "Challenge" and select the stadium you want to play in.
Once they accept your request, the Friendly Match will begin.
※ Please note that Friendly Matches do not require a gold fee to enter, and do not pay out gold for winning. Friendly Matches also do not count towards quests and achievements.