First multisig using Armory
Easiest way to do this is to first load watch-only wallets of the multisig lockbox you hope to create. Say, you want 2-of-3 and you have 3 watch-only wallets.

Go to Lockboxes -> Create Lockbox -> and click the address book icon next to each public key #1. Highlight the wallet you want the public key to be from and select an address below that hasn't been used yet. Repeat for public key #2 and public key #3. Generally, you should be choosing from different watch-only wallets.

Click Save Lockbox and you now have a new multi-sig p2sh address!