RE: Managing two or three separate companies in CE
Sorry, I´m not sure if I understand your case, but I thnik that these users could be use 'ALIAS' for their e-mail address registered as contact to receive alerts from cases and on the other side the end user should 'reply as' that ALIAS (best as 'Group' even this contains only his email access/account).
In that way exemplified, 'Mr John' can work for company 'a' as 'john@a-domain.com' but also be registered as a contact inside Dyn365 as 'John@b-domain.com', so the same user can manage "02 company's cases" on the Dyn365.
I appreciate if you could examplain a little bit more your case. We are MS partner for IT Infrastructure and new to Dyn365 but looking to improve our knowledge also helping and learning at the same time with others here.