You can search for a contact and see list of spaces you share. You can either use the existing spaces or create a new space. You can also call them directly.
Chat messages sent to a space are visible to space members. If you wish to have a private chat with one person only, create a space with you and the person to exchange messages. You cannot chat with someone outside a space.
During a meeting, you can chat with someone. However, chat messages are deleted at the end of the meeting. Use spaces to communicate, if you wish to retain records of chat messages and calls.