The easiest way to do this — and the main way this is meant to work — is by creating a group and inviting your RAs to it.
At the moment that will limit you to using stored files, since linked files aren't currently supported in groups. When per-group base directories are implemented, we'll allow linked files in groups and you'd have the possibility of storing the files (not the database, as in your other thread!) in Dropbox and setting a base directory for the group.
But if your RAs aren't using Zotero themselves and you're comfortable sharing your password (as you say in your other thread), you can all use a single account with a single personal library, and then you could use linked files and a base directory in Dropbox now. But that's not the recommended approach, since it requires sharing your account credentials and prevents the other people from (easily) using Zotero themselves.
Any estimate of when that functionality will be available?
No, sorry. It probably won't happen without a patch being contributed by someone in the community (which is how base directory functionality was initially implemented).
Upgrade Storage
At the moment that will limit you to using stored files, since linked files aren't currently supported in groups. When per-group base directories are implemented, we'll allow linked files in groups and you'd have the possibility of storing the files (not the database, as in your other thread!) in Dropbox and setting a base directory for the group.