You'll be able to view all the Friends you've added in Bingo Story in the "Friends" section of your profile. When you open this tab up, any player in your club will be displayed in this section as well. You can add them as friends here if you wish.
To add a friend in Bingo Story that's not in your club, simply click on their profile image or name to bring up their profile.
You'll find an Add Friend button here. You'll have to wait for their approval but this is the easiest way to add friends in Bingo Story.
If you have another player's User ID, you can search it by clicking the "Find New Friends" button in the "Friends" section of your profile.