There is no central authority -- it's fully decentralized. One person/device prepares the transaction, and whichever device adds/imports the last signature can broadcast it. It is passed around as ASCII-armored blocks of text that can be emailed, and/or copied to USB to take to offline devices. We
would like to add a central server that would help users pass the data around, further improving the usability for those who want it... but the decentralized solution would always be available.
P.S. -- This is what people pass around to collect sigs (either inline in emails, or save to file):
=====TXSIGCOLLECT-4ogNaap7======================================================
AQAAAAsRCQcAAAAAAf1OAgEAAAALEQkHX/rr6vit6h6hFEtrSevRUWb0ev79VNLukkIjNUNoZj8AAAAA
/QABAQAAAAEGOwaI7hG2eALnVRhP5U9htkL5nR7pkmuhA++lkvmwjgAAAACLSDBFAiALPCTF3jPh238+
0TRiv7078ukWSykh5etoOC92mgHUtgIhANoaevyjicUcXZrBG4B05MCe0b6Ao4oQF1MWratKk51XAUE E
ziXsxcDUeorfEALx23AQ7/850SCxb/a+ZxbqIQ48ExhQgyCnQRF8ljIqOHz+5p7nJrWdfhVy2vtwICfz
vFbMff////8CQHh9AQAAAAAXqRTllz72+r9S3h22NtdNbPS0W/TqhofAcCc0AAAAABl2qRRJvTihhk4j
NbkfeRLljm0F7x58aYisAAAAAIdSQQRaVaMnjUAAEwZ8WtKXeCYt1UlDBNmX3woLIHwh2+sDB8xs5nqY
6iygkHrJZMSUJncnX/g29PgFrv9Sco2BZKh/QQSTtOSfxz3CHvwktSKnps0SiTZwzoLBqyuju1BawtJn
5FvVUfOSJGjflmvJVB8A3Kk6IIbdW14wv8/YpL4yZpfsUq4ISHVla2pwcEwA/////wJBBFpVoyeNQAAT
Bnxa0pd4Ji3VSUME2ZffCgsgfCHb6wMHzGzmepjqLKCQeslkxJQmdydf+Db0+AWu/1JyjYFkqH8AAEEE
k7Tkn8c9wh78JLUip6bNEok2cM6Cwasro7tQWsLSZ+Rb1VHzkiRo35ZryVQfANypOiCG3VteML/P2KS+
MmaX7AAAATQBAAAACxEJBxl2qRTAJVPUOGJ3S2BllgR6HNc32I8NloisMFF9AQAAAAAAAAROT05FAAA A
================================================================================
A standard text format for "sig codes" that can be sent between users is what multisig needs.
Can this become a BIP? Something like this can become a standard that all the wallets and other services can support.
But, ...
Would it be better to use JSON clear text, so the user can see what tx/out/etc the sig refers to? Otherwise, users will get several sigcodes mixed up and go crazy.