Don't pass func_get_args() directly to apply_filters() in groups_get_group_members()

PHP 5.2.x does not allow this syntax, so we have to put the value into a
variable first.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7154 cdf35c40-ae34-48e0-9cc9-0c9da1808c22