Hello,

All users can change company, to what they belong. On login screen, when they see company with credentials to enter, press cancel, and users will then see the company list window, to select the company they belong to.
On second login, the system remembers last login company.

Every user should have standard ID in their persons card.
Please check in System>Persons> for every user, that standard ID field has their emails, not the same for all.

Best Regards,
Hansaworld support