Hey Paddy,
Team System Licensing is hard work at the best of times...
The "retail" scenario would be for you to purchase a TFS License and then make sure you have TFS Client Access Licenses (CALs) for everyone needing to connect to TFS. You get a TFS CAL included with any Microsoft Visual Studio Team Editions you might have. If you just all have Visual Studio Professional or Standard then you'll need to purchase CALs.
If you are a certfied partner with ISV competancy then you qualify for a free copy of TFS (you do if you are a Gold Certified Partner as well) and you get a discount off TFS CALs.
The easiest thing for you to do is probably to contact the MS Ireland licensing team (See http://blogs.msdn.com/irelandlicensing/) and click on the "Email" link at the top of the page. They are a very friendly bunch and know all the deals to be had (I'm pretty sure you can get good discounts when getting more than 5 of anything at one time from Microsoft).
Cheers,
Martin.