I learned (the hard way) that joining a workgroup while on a domain removes you from a domain. I need the computer connected to the domain so it can connect it's internet, but I would like to connected to the workgroup for file/printer sharing, etcetera. To re-join the domain I need administrative privileges (on the domain) which I don't have.
Simple answer is that you can't - the system is either a domain member or a workgroup member.
Long answer is that you probably don't need to. You can probably get away with just keeping it on the domain.
I have never had any problem accessing shared resources hosted by non-member systems from a domain-joined system; that is to say, you don't need to be a member of the workgroup in order to use its shared resources - you just need to have valid credentials for the non-domain server you're accessing.
Likewise, I've never had any trouble accessing shared resources hosted on domain-joined servers from non-domain systems; typically, you just need to provide domain-based credentials when accessing the particular resource. For instance, you could use this command line to map a network drive hosted on a domain member server from a non-domain client system:
You can use this approach with graphical tools as well.
I can't guarantee this sort of thing will work in all network configurations, but it's always worked for me.
Upon joining a domain, your system uses the domain for things such as authentication and group policy. The way Windows accomplishes browsing is by essentially using the NETBIOS version of the domain as a workgroup.
If you are able, you can give the non-domain workgroup the same NETBIOS name as the domain (so if your domain is