Ensure that 'page' value passed to BP_Groups_Group_Members_Template is respected

Previously, it was being ignored - the only value considered was
$_GET['mlpage'], with a hardcoded fallback of 1.

See #921

Props imath

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7931 cdf35c40-ae34-48e0-9cc9-0c9da1808c22