1..1 and 1..n cardinalities on the same relationship
In your requirement employee means not boss, so this should work fine.


-- Company (CompanyID) exists
--
Company {CompanyID}
     PK {CompanyID}

-- User (UserID) works for company (CompanyID)
--
User {UserID, CompanyID}
  PK {UserID}
  SK {UserID, CompanyID} -- superkey (unique), needed for FK ..

 FK1 {CompanyID} REFERENCES Company


-- User (UserID) is boss in company (CompanyID)
--
CompanyBoss {UserID, CompanyID}
         PK {CompanyID} 
        FK1 {UserID, CompanyID} REFERENCES User



Note: all attributes NOT NULL