When you have opened an instant session window, you can invite further contacts to join the session using the Invite button in Session Contacts area of the instant session window (see Creating Sessions).
Clicking on the Invite button displays the Contact Selection window:-

Contact Selection Window
1. Contact Folders - you can filter contacts by contact folder by selecting from the treeview to left of the window.
2. Contact Selection View - selecting a contact folder in (1) will display the corresponding contacts in the contact Selection window. The contact name and the current state of the contact is displayed in two columns. If you want to talk to the person, you will be looking for contacts with an IM account who are shown available.
3. Search field - type the contact first name or last name in the Search field to select the contact in the Contact selection View below.
4. First name, Last name - click on the First name button to sort contacts by their first name, or click on the Last name to sort contacts by their last name. You should type a value in the Search field depending on which button is depressed.
5. New - create a new contact using the new button, and New Contact Window displays for you to fill-in the person's details. (see Creating Contacts).
6. Open - alternatively select a contact from the Contact selection window, and click on the Open button to edit the contact details.
7. Cancel - use the Cancel button to cancel any selection, and return to the instant session window.
8. Select - use the Select button to invite any selected contacts made in the Contact Selection view above to the current instant session window.