I believe you can create a group in your organization, then invite whoever you want to that group, whether or not they have their own organizational account. So if you have an organizational account already, create a group, then invite only those people you want to the group using their email address. I've invited several people to my group that don't have their own organizational account. after you invite the users you want, setup the group permissions to only that group.
I just invited my wife (who doesn't have an organizational account) to my group, she was able to join and start making maps with no problems.
This is a really old post,
But check out how to share across organizations using this blog:
The system doesn't allow someone with a public account to join a group owned by someone with an organizational account, nor vice versa. We never allowed this, as far as I can recall. However, there may have been a bug back in 2014??
Mike