building a multiple network multisite with a common theme

Hey. So my design idea is to create an ecosystem, in which multiple networks (buddypress) would have a common user database that they would share, where a being could log into one (which would have it's own visual front-end) or another (same, unique front-end) and be able to move around in the ecosystem.

I want the various networks branded separately, with distinctive visual designs, so that it's cleaner to work with for users & Authors.

The three main components i'm envisioning at this time are:

1) Into the Mythica

2) Djedi Academy

3) Circle Moon

All within the "Mythica" ecosystem.

I want ITM (Into the Mythica) to be content-specific, in this case, relating to Authors of Stories.

I want Djedi Academy to be content-specific, in this case, relating to workshops & classes in the yogic arts.

And Circle Moon to be content-specific, in this case, relating to people, products & media regarding the natural World and the Goddess.

It would be nice to have each of these in their own network; connecting within a larger ecosystem. That way, beings who are interested in 'Djedi Academy' could focus there on the healing arts, whereas Authors with 'Into the Mythica' could focus on their Stories; and the general populace of individuals relating to the theme, which is the natural world; would converge in 'Circle Moon'.

I'd like to have a Marketplace that works within this ecosystem as well. One in which beings would be able to create pages that had Marketpress or WooCommerce capability, feeding a percentage back into the ecosystem.

So, in essence, users would exist in their own environment (ITM/Circle Moon/Djedi Academy) and would *also* be part of a general database.

The Marketplace would work throughout the whole system, allowing classes from Djedi Academy, crafts & products from Circle Moon, Stories from ITM; all working within the ecosystem.

Now, i've got a multisite for 'Into the Mythica', and a separate multisite for 'Djedi Academy' and 'Circle Moon'. Is there a way to keep everything within a single multisite or share users between the sites but maintain distinction as well as share a common marketplace?