Foursquare has added a new app feature that it calls its "most requested": the ability to check in your friends for them. Foursquare already has a mention feature that lets you tag the friends you're with, but it has never before allowed you to use those mentions to check in for others on their behalf.
Here's how it works: When you open the "I'm with" feature as you're checking into a location, you can choose the appropriate friends and Foursquare will send those friends requests asking for permission to let you check them in. Once you grant someone permission to check you in once, they can continue to do so in the future.
This update is an interesting return to Foursquare's roots as a check-in service--its last few app updates have focused more on the tool's personalized recommendations section, Explore.