This is an useful snippet that will add your newly registered (via WPMU DEV Membership 2) member to corresponding BuddyPress group.
You can add those codes in your functions.php in the theme, if you think your theme wonâ€™t be changed. Otherwise mu-plugins is the best solution. To use mu-plugins, go to /wp-content/ and find the folder with name ‘mu-plugins’. If there is no folder in that name, then create a folder, name it ‘mu-plugins’, create a file inside that, give any name you like and paste the code in there. You don’t need to activate that plugin. Mu-plugins means must use plugins, so it will be activated automatically always. If you use mu-plugins then add a php start tag at the beginning of the code.