The development is done by username Naatan (Nathan), who is an experienced developer.
Basically, he will hold the copyright to the add-on, otherwise the cost would be double the above, close to $4k. This enables him to sell the completed product as an addon. If he sells it with annual updates, etc, or increases prices as others contribute to having pieces added to it, as is uses an open source authentication protocol, OpenID, to communicate between installs, regardless of server, by a whitelist of domains, then we benefit by accessing upgraded releases as part of our higher costs.
Those who contribute to the development, basically Nathan has stated he is happy in doing the right thing with those who get the product launched, compared to what will be offered to those who buy the addon at a lower price once developed, ie. they may have to pay an annual fee for upgrades, upgraded costs for additional features added, etc.
What that becomes... will more be dictated as the product develops. Using OpenID allows for any software to simply have a slave built and added into the equation. No doubt people may have a master built for say, WordPress, and use the XF installs on slaves only. Who knows.
The initial build is only XF to XF, no limitations on quantity. One master, the rest slaves.
If a member registers on the master, then their account is synced to the slaves and vice versa... if they register on a slave, their account is synced to the master and other slaves, hence, single sign on solution. It has conflict resolution built-in as well... especially for the initial sync.
Each site manages permissions uniquely, profiles, etc... as this is about user accounts at this stage. Later on, someone may want to further permissions development, redirection signon, all sorts of options.
As stated... as it develops, those who contribute get updated. Those who pay for an XF to XF addon, will likely have to upgrade to get access to further development benefits... Once built, Nathan may then expand the addon at his own doing and up the cost of the addon to those who purchase, if they want additional features. Those who contribute the higher costs now to get this developed, then reap some rewards later down the track with ongoing updates. If XF change something that would break the solution, we don't have to pay for the minor changes to be made, etc etc...
Basically, Nathan is about doing the right thing by us once developed... as we're paying him to get it developed, and this is his full-time job, being software development.