How to setup a External HDD to a workgroup

[m16si]

Hello i have a problem.

I want to setup an external HDD into a workgroup, so that 4 computers can access it through a workgroup and share informations and data with it!

I have 4 laptops that work on wireless internet.

And three computers have Windows XP and one has Windows Vista.

Could you please describe the whole procedure because i really don't know where to start.

Thank you very much...

m16si