As the title says. If Company A sets a user to be have the UserID of JohnDoe, Company B (a completely separate entity and customer of yours) cannot create a user with the UserID of JohnDoe.
Change the Primary Key from UserID to the email address since that will be different from company to company.
John Doe, JohnDoe@CompanyA.com works for Company A
John Doe, JohnDoe@CompanyB.com works for Company B
We have received this request before, but we generally recommend that you just use your email address as your UserID. This cleans up most of the issues and seems to mimic what a lot of other online companies are doing.