Go to Server Administrator. Click on the name of the server in the left-hand side (not any particular service, just the name of the server). Click on (Shit, I'm doing this from memory...) Uh, I think it's "shares?" And then click on your share in question. the bottom half of the pane should have some little tab headers you can click, I think one says Permissions? Add an ACL that permits Read & Write for your group-in-question, and forces inheritance for all contained files and folders. Hit the Save button on the bottom. THEN hit the little gear icon thingee near the bottom leftish and do 'propagate permissions'. The last step is to make all contained folders have that new ACL, the first step is so that all future created files and folders have the read-write permissions set OK. The ordering of those last two steps probably doesn't matter.
It's not particularly complicated, but it's not intuitive, and it's not what the help files were saying. Nor what people in various forums were saying. I'm doing my trick of documenting it here so I don't have to look it up next time :P