In civivolunteer, is it possible to allow multiple users for 1 email?
The short answer: Yes, it's possible.  

You're asking about the "Unsupervised Dedupe Rule" - that is, the test to determine whether a contact who fills out a public-facing form is a duplicate of someone already in the database.  

Documentation can be found here, and I definitely suggest reading it before making changes.  However, the general idea is to make up a different dedupe rule (e.g. "same email AND same first name" might be better in your case).  Then go to Contacts menu » Find and Merge Duplicate Contacts, create the new rule, and make it the new "Individual Unsupervised" dedupe rule.