Click "Chat" on the left hand side.
Click the "New Chat" button:
In the "To" field, add each member by searching for their name and selecting them.
If you wish to give the group a name, in the far right of the "To" field, click the down arrow button:
In the "Group Name" field, enter the name you wish to use.
At the bottom of the screen, type your message and press enter.
Please note: all chat messages are retained permanently and cannot be deleted by users.