To add another user as a friend, click on their avatar and then click on the 'More' button on the right hand side of the screen. When their full profile pops up, choose the 'Add Friend' button in their profile. 

They will get a friend request pop up, and once they have accepted the friend request, they will appear in your 'Friends' list. You can access your Friends list by clicking on the 'Friends' tab at the bottom of the screen.