Figuring out how to get everyone's number at once without looking creepy or putting anyone in particular on the spot is left as an exercise for the reader. ;)
(The first one I'd try: a friendly group contact info exchange suggestion, to be able to keep in touch, would let me then follow up one-on-one later over text/calling/facebook/your method of choice, without having to corner anyone.)
There's that whole set of people that don't use Facebook ... also, creeper factor much? "hey.. I was searching for you on facebook, and... wanted to know if we could hang out" insert heavy breathing where most awkward to fit my mental image of someone stalking me on facebook
Maybe that's the key to making a good product in this space. Making a group connect web page for each get-together putting an optional amount of contact info in it after the get together. Also add an IM system.
Looks like you need to have Facebook to sign up, which I assume is how they screen people. Unfortunate, as I dropped my account a few years ago and I would have tried this, looks like fun.
You are probably right if you are adding someone with any mutual friends. If you can frame it more as "I saw you comment on x's status so I added you" that is fine.
(The first one I'd try: a friendly group contact info exchange suggestion, to be able to keep in touch, would let me then follow up one-on-one later over text/calling/facebook/your method of choice, without having to corner anyone.)