I'm building a Bitcoin ATM that will use Multi-Sig for security and I realized I can't create a MultiSig Account with ONLY the Bitcoin Addresses.

Why does MultiSig Use Public Key's as opposed to the compressed 1xxxxx Bitcoin address?

I feel that most of the time I'll be building MultiSig Accounts with other people. Most of whom will NOT have their public Key. Only their private key and bitcoin address.

If you have the android client. if you have a blockchain.info account. You DON'T have your public key.

Is there a way to build a multi sig with BITCOIN ADDRESSES only? (or at least ONE public key from one person (me) and a the bitcoin address of someone else?)

is there a way to CALCULATE the PUBLIC KEY (PUBKEY) from the Bitcoin Address?

Please help!