The organization you belong to determines which people you can reach out to in a private chat, one-on-one. They have two ways to give you access:
- By subscribing you to an open chat group. You will always be able to open a one-on-one chat with another user with whom you share an open chat group (not a read-only group). Click on the title of the chat and you will see the list of members.
- By joining a team: When you are a member of a Team (e.g., your department), the Team Administrator can give you permissions to view your colleagues' contacts and start a one-on-one chat with them.
Additionally, if someone can send you a direct message (because they are an Administrator and have access to the global directory), you will already have the chat open with him/her and you can chat privately.