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)
{
...
}
}