It's possible to set things up in Fastmail so that mail sent to a particular alias is automatically forwarded to all members of a group... which is almost the same thing.
I know this because I did it myself last year, with help from others on this forum.
See this thread:
http://www.emaildiscussions.com/showthread.php?t=74101
You will find Bill's contributions to that thread massively helpful if you want to go down that route.
I also added some Sieve to check before forwarding that incoming mail received at the alias was sent by a member of the group.
I'm a bit hazy now about the details - but if you think it might be useful to you I'll dig out my code and post it here.