The VM Guest Unmonitored group is used internally by the MP, to disable workflows during certain discovery processes. This group disables some workflows for VMs while our discovery process is going on, so that these workflows don't try to run on the RMS, which could cause some errors.
The group is part of our sealed MP, so you can't modify it - there's no need anyway, because VMs automatically join and leave the group, controlled by our discoveries.
I agree - this group could be useful if you could add/remove VMs manually, and so disable alerting and data collection per-VM. (similar to Ops Mgr maintenance mode, but actually this group disables ALL data collection, not just alerting).
We could include an unsealed MP in our Resource Kit that would give you a copy of this group, for your own use. Then you could add and remove your own VMs as needed. Let me know if that would work for you!