I verified following RA. To set agents to agent groups was required to use as retrieval agent on all version. So, it seems this behavior is not changed recently.
If an agent is belong to Agent Groups on ROC or ASAP, I could see the agent in Agents list.
On the other hand, I found one different behavior between RA 6.5 (or later) and 6.4 (or earlier) in my verification.
I don't know the reason at this stage.
On RA 6.4, I could select agent to create new group on ROC even if the agent is not configured in Automation Studio.
On RA 6.5, I could not see agents in ROC when agents are not existed in Agent Group in ASAP.
Currently, agent groups defined using ASAP or ROC are mixed. According to our plan, all of ASAP features will be moved to ROC. So, I suppose side effect during the transition.
> Agent Assignment: Only Agent Group defined by ASAP
> Retrieval Agents: Only Agent Group defined by ROC.