Remember to make it visible?
(Just tested on 5.0.9_GA_2533.RHEL5_20080815101705 CentOS5 NETWORK edition)
It was implemented far back in 5.0 betas:
Bug 12212 - Family Mailboxes
For briefcase you need 5.0.7+:
Bug 28149 - offline : Briefcase application tab remains disabled for multi account
Docs you need 5.0.9+:
Bug 28411 - family mailboxes : documents & briefcase disabled