Is it possible to asign an issue / task from project A to a employee that is not listed in project A?

[Nic Brough -Adaptavist-]

To be assigned an issue, a user must match the rule for "assignable user" in the permission scheme.  That usually means that they're in a role for the project somewhere, but doesn't has to.  I'd make sure it matches the "browse" permission too, or you'll be giving issues to people who can't see them.