If you are an Account Administrator, only you and others with the same role as you can see the list of all chat groups in your account. You can see them in the Settings > Groups:
In the list, you will see all the chat groups of the organization. If your user does not belong to any of those groups, you will have the possibility to join by clicking on the "Join" button:
Please note that members of open chat groups are always public: if you join an open group and other users from the same group check who the members are, your name will appear as long as you are part of the group.