Ignoring for the moment how much trouble the Windows default permissions can be (an Everybody group? Really?), all you really need to do is tell Win7 you have an Workgroup of "X", then tell XP you have a Homegroup of the same name.
To add to this, unless you're running an Active Directory server (which I hope you aren't. PITA) you'll need to enable write permissions once they're sharing. You can have a 'ShadowRaven' user on both computers, with the same passwords and they won't be considered the same account. Windows will see 'XP Computer/ShadowRaven' and 'Win7 Computer/ShadowRaven'. The easiest (although horrible security) is to give the 'Everyone' group write permission.
The alternative is to manually ensure that each Computer has the
other computer's user in it's own local permission list.
But that's down the road- you need to get them sharing first.
You can 'share' a folder by right clicking it in Windows Explorer (Right click 'start' and select 'Explore' or 'Open Windows Explorer') and then selecting 'Properties'. Next, select the 'Sharing' tab and click 'Share...'
Note that one of the ways around manually configuring permissions or adding the 'write' permission to the Everyone group is to do away with the Workgroup/Homegroup thing entirely.
Instead, share the folder you want on the computers, then 'map' the share as a 'network drive' on the other computer. This would use the local credentials (you essentially log into the other machine) and it's the way I, personally, would go about it.
It requires you to manually set a static IP address on each computer though.
