You can disable this notification when someone joined the group by following the steps below:

    1. Navigate to the BuddyBoss dashboard - Settings - Activity.

    2. Under Posts in Activity Feeds, untick the "Members join a group" option. You may refer to the image below:

    3. Click on "Save Settings" below the page to apply the changes.

Screenshots:

Here's what the notification looks like on the news feed: