To start a conversation with a single co-worker or contact, go to ‘Contacts’ or ‘Coworkers’ in the menu on the left side of your screen. Look up the person you want to chat with. Click on that person and a conversation is automatically started.
Is the person you want to chat with not in your list of contacts or coworkers? You can easily invite new colleagues or contacts.
Go here for instructions on inviting co-workers or external contacts: