I'm currently working on a silverstripe site whose logic will be rolled out to multiple sites on multiple domains. Basically I want that when a user signs up for one site they can automatically sign up for others with a simple opt-in. Is there an easy way to share Member Data between the sites, or is the best way to write a function with Vanilla SQL writes to the other domains?

I don't suppose you are using the subsites module to power the multiple sites / domains? then they can share the 1 database which would solve the syncing issue.

Hmmm, I haven't had a chance to play with it yet, I didn't realize it would work across multiple domains as well as sub-domains. I can see it solving a few bits of feature creep that have arisen, but one of the sites will require a bit of customization to my base code which I don't really want to roll out to the others as to not overwhelm the content creators. Its really only the Members I want to share on an opt-in basis

I'm also a little confused by the requirements. It lists GeneralDataAdmin and SS 2.3 as a requisite even though GDA is depreciated in 2.3. I was considering implementing it manage Mobile version of the sites.