Now add an else condition where you reset the visibility to invisible

import android.app.Activity;
import android.os.Bundle;
import android.view.ListView;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.GridView;
import android.widget.TextView;

public class InviteActivity extends Activity {

    /*
     * Some code has been omitted for brevity!
     */
    public MenuItem mInviteMenuItem;
    public GridView mGridView;
    public TextView mEmptyTextView;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.player_grid);

        mGridView = (GridView)findViewById(R.id.gridView);
        mGridView.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);
        mGridView.setOnItemClickListener(mOnItemClickListener);
    }

    public OnItemClickListener mOnItemClickListener = new OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

            // Start by adding code in here!

          if (mGridView.getCheckedItemCount()> 0) {
            mInviteMenuItem.setVisibility(0);
          }
          else{
            mInviteMenuItem.setVisibility(1); }

        }       
    };
}