Storing the PST on the network in a workgroup or domain is NOT RECOMMENDED and NOT SUPPORTED by Microsoft. You risk loosing the mail in a corrupted file. Further, the user will start getting multiple copies of the same e-mail.
Profiles are based on SIDs - with no common domain to unite the users, there is no way to create a roaming profile. You may be able to rig something with registry or local policy edits, but I would consider that VERY UNWISE. Microsoft Foundation server costs $300 (though it is only available with new servers) and supports 15 users in a domain configuration. You may need to upgrade your clients from home versions (if that's what any of them use), but really, I've got clients who have ASKED me for a domain with JUST 2 workstations... and there's no way I recommend a Workgroup with 10 machines - indeed, you are on the upper limit of shares - add an 11th and you'll start getting conflicts because on Vista and XP, you cannot have more than 10 simultaneous connections (with the business class products; 5 connections with the home products)