When you check in to a place or comment on a check-in, you can ‘mention’ one or more of your friends. Start typing your friend’s name and you should see their full name and profile photo pop up in the bottom left corner of the shout box. Tap the box to ‘mention’ your friend (if he/she is friends with the person who checked in, this will send them a notification that you have mentioned them in your check-in). If you mention your friends and share the check-in to Twitter, their Twitter handles will automatically be added in place of their names, if they’ve already linked their Twitter and Foursquare accounts.
Note: when you mention a friend, you are checking him/her in if he/she has ‘check-in by friends’ turned on. Likewise, when a friend mentions you, he/she is checking you in if you have ‘check-in by friends’ turned on.”
“If you, or your friends have ‘check-in by friends’ turned off it will just show up as a mention. When a friend ‘mentions’ you in his/her check-in, his/her friends can see your name and tap through to see your public profile. You can opt out of ‘check-in by friends’ by going to ‘My Settings’ in your app, then ‘Account Settings,’ and ‘Privacy Settings.’ You can opt out of being mentioned on Facebook or Twitter by going to ‘My Settings’ in your app, then ‘Account Settings,’ and ‘Privacy Settings.’ Uncheck the box next to, “When my friends check in with me, it’s okay to include my name on their check-in tweets or Facebook wall posts.