How can I change users permalinks from "site.com/members/username" TO "site.com/username"
Please use this snippet code
define ( 'BP_ENABLE_ROOT_PROFILES', true );
add_filter( 'bp_core_enable_root_profiles', '__return_true' );
Put it on bp-custom.php file on wp-content/plugins folder. If you dont have it yet, please create one.