Its not cross domain its all on the same site. mydomain.com/group/
but to become a user for the social site you have to be part of the group. So you would register with the group.
I think you mean like MultiSite http://community.elgg.org/pg/plugins/alivin70/read/384902/multisite-pack I'm not sutre what development is going on... check ou their group http://community.elgg.org/pg/groups/21771/multisite/
@MB
MultiSites is quite irrelevant because it does not address the crucial issue that @TD is looking for.
@TD
"Cross-Domain" Vs "Non-Cross-Domain" does not matter ;-) The technology will be the same..
Register on one site "A" gives your users the ability to login to site "B" - where site "B" happens to be elgg-based. This has been done already - as you can see via the Moddle-Elgg interfacing I mentioned above.
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