To add other users to a conversation you are already having, follow these steps.
Alternatively, you can start a group conversation by right-clicking the name of the group in the user list and then clicking Start Group Conversation.
A list of users taking part in the conversation will appear on the right. To quickly insert the name of a certain user to the Message text box, click it in the list.
To leave the conversation, click Close, and then click Yes in the dialog box which appears next.