There are two ways you can join friends:
1) From your friend's list, you can scroll to where your friend is at in the list and use the join button.
2) If you have a long friend's list, #1 above can be a bit cumbersome. As an alternative, you can go to the map and choose to join a game. Then you can filter for your friend's name and quickly join their game. If you're grinding and your friend is remaking, the join game can be a quick way to get back into it!
Have fun!