Can a user register in any of the subsites?

Hi,

A customer wants 3 online stores that shares the users. That it, that when a user register in a store he will use the same user account in all stores.

I think that this plugin can do it only from the master site to the slaves. But, is there any way to sync the users independently of the store he has registered to?

I've found this plugin also: http://wordpress.org/plugins/multisite-user-management/

Any idea?

Thanks.