Questions About Multisite and Users

Hello to everyone!

I have some questions about MarketPress multisite!

I have a multisite with a central site and multimple sub-sites.

Every site is a store. At MarketPress I disable the global cart, so each customer finish his shoppers to each store.

So each customer should register to do shopping.

So my question is : Does the customers should register to each store, or can they register once? How can I do it?