Can multiple people create one multisignature address, while keeping their private keys private?
You need the public addresses of all persons involved in order to create the multi-sig address. So if you are creating a 3 of 5 multi-sig address you need all 5 public address.  In order to retrieve funds from that address you need signatures from 3 of the 5 people (or however many signatures you require). Check out this example here, which was a huge help for me. 

bitrated.com and https://coinb.in/multisig/ have open source implementations of this as well.