That would definitely be an awesome feature, and we've considered adding it in the past.
To be honest, it would be a complex addition to the codebase, so I can't say that we will be tackling it any time soon.
The three of us (the contributors to the plugin) work on it in our spare time or when we have a client who needs something specific with metadata, so we're not necessarily spending a bunch of time working on it.
That being said; if you are a developer or are able to code in php, please feel to contribute. The best way to do so is clone our Github repository and submit pull requests or just post the code here on the forums with pastebin or something similar.
With all that said, we'll definitely consider adding in multipliable groups in a future release, because I also think it would be pretty killer!
Thanks again for your feedback!