Wow, looks like you and your friends need something else to do with your time. Did you think at all about donating your time to a charity? :-)

No, that's not fair. My friends and I compare who signed up when for social networks too. But then again, we don't always have the smartest allocation of our time either. 'nuf said on that topic, okay?!

Back to Foursquare.

There are two ways you can figure out your account number, which, since it's presumably assigned incrementally should quickly answer any questions of who signed up before whom, should do the trick for you.

The first way is to just log in to Foursquare.com, click on "Settings" and look on the right where it shows all your account information. Here's mine:

As you can see, I have Foursquare account #12680, making me a fairly early adopter, I suppose. :-)

You can also figure this out from within your iPhone Foursquare app too. Go to your account information:

Tap on the gear button on the top left to get to your settings:

And tap on "Account Details" to find your account number:

Now, borrow your mates phones and do the same, then see who was there first. Good luck!