[Sarah Lewis]

Replying well after the fact to help any other Google searchers who come across this page:

Add something like the following code to your theme’s functions.php file:

add_action( 'bp_members_directory_order_options', 'bp27845_add_random_order_option' );
function bp27845_add_random_order_option() {
	echo '<option value="random">' . __( 'Surprise me!', 'buddypress' ) . '</option>';
} // end bp27845_add_random_order_option

This code takes advantage of the bp_members_directory_order_options action hook to insert a new option, which then sets bp_has_members()‘s type to random (the value we’re sending).

A more in-depth tutorial: https://codex.buddypress.org/plugindev/add-custom-filters-to-loops-and-enjoy-them-within-your-plugin/#add-a-custom-filter-to-the-members-groups-and-blogs-loops