The Chat tab will display the list of chat groups that you are part of and all the open and closed groups that are in the spot but you are not a part of it yet. You will not find any private group in your group list as these are hidden groups.
To join a chat group, follow the steps below:
Open group
1. Go to Chat and click on Join Chat Group
2. Click on the Join button to become a part of the open group 
You will now have access to the group
Closed group
1. Go to Chat and click on Join Chat Group
2. Click on the Request Join button to become a part of the closed group 
Once the admin has approved your join request, you will have access to the closed group
Private group
The only way you can be a part of a private group is when the admin adds you to the group
When an admin adds to a group (open, closed, or private) you will be notified via an email, SMS, or app notification.