When you check in on foursquare to a place whose mayor is your friend, your check-in message is "Your friend X is the mayor here (and you're creeping on their turf)."
What does this expression mean?
In foursquare's game mechanics, the person with most checkins (in different days) to a venue is the venue's mayor. So, when you check in somewhere you're actually getting yourself close to steal that person's mayorship.
I understand that the word "turf" here has to be a slang for territory; my main doubt is about the word "creeping," then. Does the sentence means "you're on your way to steal your friend's mayorship" or "you're enjoying the benefits of hanging out on a place that is commanded by your friend"?