I would like to know about what's best practice or standard process before and after adding new system into domain?
Best practices will be proceed with anti virus scan on the machines before allowing them to join to the domain. Make sure virus definition and standards meet your organization requirements.
And once you have joined the Domain, make sure your security policies are tight and monitored regularly.
Domains, workgroups, and homegroups represent different methods for organizing computers in networks. The main difference among them is how the computers and other resources on the networks are managed.
Computers running Windows on a network must be part of a workgroup or a domain. Computers running Windows on home networks can also be part of a homegroup, but it's not required.
Computers on home networks are usually part of a workgroup and possibly a homegroup, and computers on workplace networks are usually part of a domain.