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Unfortunately, this is quite complicated. Adding another item to the Order By dropdown is quite easy, using the ‘bp_members_directory_order_options’ hook (or putting the additional option directly in the members/index.php file of your theme). But making it actually work in terms of reordering will be complicated – it will involve hijacking part of the loading process, and filtering the members queries (see bp-core/bp-core-classes.php, around 193).