All done

src/ParticipantsArea.java

+import javax.swing.*;
+import java.awt.*;
+import java.util.Vector;
+
+public class ParticipantsArea extends JPanel {
+
+    private Vector<String> vector ;
+    public ParticipantsArea(){
+        super();
+        vector = new Vector<>();
+        vector.add("ALIREZA1");
+        vector.add("LOOKI2");
+        vector.add("MOHAMMAD3");
+        JList<String> jList = new JList<>(vector);
+        jList.setBackground(Color.GRAY);
+        setLayout(new BorderLayout());
+        this.add(jList , BorderLayout.CENTER);
+        JLabel jLabel = new JLabel("Online users : ");
+        jLabel.setBackground(Color.red);
+        this.add(jLabel , BorderLayout.NORTH);
+    }
+}