You can challenge someone to a friendly by going to team pages and click challenge by their stadium. If challenge is grayed out, then either they or you already have a friendly scheduled for that week. An easy way to find those who are looking for friendlies is to go to "Community" above, then "Ads" to the side, then "Friendly Game" tab near the top. You can also post there yourself. You can only make 1 post per week (I think), so you might want to wait until Monday to post for next week. Hopefully, you can find one looking this week still. Good luck.