This setup for wordpress page level setup and does not have any impact for group.

By default user can assign themselves to a group from settings page of the plugin. See https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/group-messaging/

In your website how users assign themselves to a group from signup? Did you change any code for this plugin?
Which group you want to hide from your members? Are your users logged in when they signup for groups?