To wrangle up your friends who are already on Foursquare, tap the drawer tab at the top left of your screen (it looks like 3 lines.) Tap 'Add Friends' toward the bottom of this display. From here, you can add friends through Facebook, Twitter, your phone's address book or by searching directly for a friend's name.
To add Facebook friends who are not on Foursquare yet, select 'Find Friends From Facebook.' Scroll down past your list of friends who are already on Foursquare until you see a list of names with the button to 'Invite.' This will send an invite out through Facebook for your friends to connect on Foursquare. More friends! More fun!
Need to a remove a friend?
Tap on the drawer tab and select ‘Profile & History’. Tap ‘Friends’ to see your full list. Search the friend you would like to remove and tap ‘Friends.’ A button will pop up that reads, ‘Remove as friend.’ Go forth! And clean out yo closet.
For more information, check out our FAQ all about friends on Foursquare, here.