Hi Neeraj
I think that User Group is the group you create in MSMP for Agents and not SU01 user group? You can confirm this by looking at the agent rules to see if one can be based on user group.
If you goal is to avoid all BRF+ and MSMP maintenance when you get new Approvers you might need to look at multiple steps for BRF+ rule (i.e. have a DB lookup that locates users assigned to the approval role you granted them Or can look at user group assigned to them). You could then return multiple approvers as well
There have been some posts/blogs on SCN for DB look ups for rules that you could reference for your requirements
As an aside, once you get past decision tables it might be worth talking to someone with strong BRF+ skills to assist you in designing and building an efficient rule.
Regards
Colleen