Type already defines a member called Get

you have to name your get functions:

public class UsersController : ApiController
{
    // GET api/companies/GetByUser/{company_id}/{user_id}
    public object GetByUser(int company_id, int user_id)
    {
       ...
    }

    // GET api/companies/GetByPlanet/{company_id}/{plant_id}
    public IEnumerable<object> GetByPlanet(int company_id, int plant_id)
    {
       ...
    }
}