By default, BuddyBoss does not have the option but as a workaround, since BuddyBoss sends an activation email to the user during registration, if the email is stopped, users cannot complete the registration. 

After that, all the pending account activation requests can be found in the WordPress Admin dashboard - Users - Pending. 

Also, if they have a developer, they may modify the activation template and write the custom text so users understand that they won't receive any emails but the admin will approve manually.