Your upgrade path is Groove 2007-- as a previous poster noted, there is a stand-alone version.
A couple of HUGE BIG ENORMOUS caveats:
1. If you migrate your existing Groove account over to Groove 2007, it will completely disable your Groove 3.x account. You _CAN_ get it back by re-activating (like you did when you FIRST got Groove), but then that deactivates your Groove 2007!
There is ABSOLUTELY no way to have a single Groove account coexist in 3.x and 2007.
2. I am absolutely unsure about the way that Groove 2007 is licensed w.r.t. the way it was in Groove 3.x days. In 3.x, your license was for YOU-- you could install it on multiple machines, provided that they were all logged in as you. So, for example, my coworker would have Groove installed on his home machine and his work machine, and they were set up to share folders, etc. That was part of the point.
In Groove 2007, I believe that you have to buy a copy for each computer, and at $250 a pop, that's not cheap!
3. Groove 2007 DOES appear to be able to participate in Groove 3.x, unlike some other reports I've read. (it worked for me).
However, Groove 2007 is unable to CREATE a 3.x workspace, so your new Groove 2007 users will not be able to make workspaces that your Groove 3.x users can access. They would have to ask a 3.x user to create a workspace for them.
4. (this is the deal killer for us) Groove 2007 is completely unable to use TeamDirection Project-- the tool that was bundled with Groove 3.0 Professional.
This is a travesty. We have a LOT invested in TD Project. I'm sure a lot of people do. Microsoft can say all they want about how the upgrade path for that is Microsoft Project Server, but that's complete shit.
Oh, and btw-- yes, there is TeamDirection Project 2007 for Groove. HOWEVER, it is NOT implemented as a workspace tool-- it is a SEPARATE tool that cannot integrate in any way with Groove 3.x.
5. Lastly, note that the links are gone to install TD Project if you don't already have it. There's a way to do it, but it's a big pain in the ass. More shit.
I guess that's enough bitching for now.
I'm not sure what to tell you. We've essentially given up on the idea of Groove 2007. We will not be upgrading to it. We got a crazy "last time buy" of Groove, so we have a few more left, but we are looking for an alternative, too.
We MIGHT end up going with some sort of Sharepoint-based system, but I dunno.
I'm VERY interested to see other people's solutions.