I have a few companies that share employees (customers). In these cases, it makes sense that the employee be assigned to both companies. This is especially true with regard to contracted employees, including some local IT.
I have no desire to turn these persons into Users, however, they do have a need to be able to look in on incidents occurring at both companies.
I would like to be able to assign these "floating" employees to both companies, so that they have access to the knowledge base articles and incidents from both companies.
This is something that is definitely on the product roadmap (no ETA at this stage). We're planning to add the ability to add multiple customers against an incident, as well as whole companies and all customers of a service.
This will allow you to quickly communicate things such as P1 incidents, and avoid duplication of incidents where customers are having the same issue.
In the meantime, what you can do is, turn on "External CC" under Admin > Account Wide Settings. This will allow you to add external email addresses to the watchlist