You have the option to create a contact book, which is an allowlist of users and channels that will be shown in the group tree. Channels that are not in the contact book will be hidden from you. Users will be visible as long as they are directly in the allowlist or in any of the allowlisted channels.
To create the contact book, select the Enabled option here and populate the contact book by moving the channels you want to the list on the left. Use the Users and Channels buttons above to switch between contact types.
To remove an item from the contact book, move it out of the list on the left.