I think the workboat COP refers to tenders as being towed by or carried on a vessel for the transfer of personnel between the vessel and the nearby shore. Provided it is fit-for-purpose, not overloaded, and subject to routine inspection and maintenance programmes, it should be exempt from the workboat requirements if that is what it is being used for.
It appears from your post that you are using the RIB as something other than a simple tender within the meaning of that definition and so I suspect you may need to be coded.
I don't know enough about group-working mode to comment.
This is a topic that you do need a reliable answer to and I would refer to an expert - a coding inspector or direct to the MCA - for clarification. My personal preference would be to speak to a friendly coding inspector, they seem able to interpret the rules to some extent and should be able to advise not just adjudicate