mojoPortal is not designed to scratch every possible itch so if you have very specialized needs you may need to build your own thing rather than to expect what you need to be already in the box.
If I were trying to achieve something like that with mojoPortal I would think in terms of using multiple sites (perhaps by folder name) to separate the groups and then the top level resellers could just be the admins of the sites and as such they could manage users within the site. But I'm not sure how well that fits your need or whether you might be better off building something yourself that does exactly what you want.
Keep in mind that most of the mojoPortal features are designed for trusted users and are not something you should let strangers use as discussed on our About page in the section "What mojoPortal is NOT Designed for".