identical user (and email addresses) on different sites in network possible?

I am about to set up a WP multisite installation for series of events on different topics. Every topic will get its own site, and there will be different events for each topic. There will also be a membership area for each site, to allow participants access to event material.

Participants must be able to register (become as user) with more than one topic. This would mean the same person (and email address) will be a user on more than one site. I am reading that this might be an issue, as the email address must be unique throughout the whole network. Is this true? And is there a way / workaround to allow for my setup?

Thanks!