this might be a fairly easy or even stupid question.. but

Hello,

this might be a fairly easy or even stupid question.. but how does logins on a multisite work exactly.

I am currently working on a network with marketpress installed. Is it possible to have customers for one store, use the same login credentials in every other store in the network?
And to have an Admin of a site within the network, use his login on another site in the network but with subscriber/customer permissions so he could purchase products?

And what would be the best way to have users and admins login? Always via the login of the main site, or are there other solutions/possibilities?

Thanks in advance!
Greetings,
Peter