The trouble with Flooing is that by the time Umbridge is in residence, it's controlled by the Ministry. Plus Dumbledore tells Harry that the Order have more efficient methods of communication.
I'm looking specifically for how Snape (not SEV :p ) finds out that Sirius is at Grimmauld Place and not being held captive by Voldmort and how he tells them all that Harry and Co. are at the Ministry.
I've been mulling over how he'd contact Dumbledore and I wondered if he could still get in to his office and maybe use the portraits or else Fawkes is involved somehow.
Any help appreciated
My guess is that Snape could still enter Dumbledore's office - he surely would have known the password. He then could have used the portraits - Phineas Black, for example, to check in at Number 12.