On "Members" page, use group loop instead of manually creating a group object

I know that BP 1.7 is technically superseding this plugin, but I thought I'd send a pull request anyway.

The "Members" page uses the `'bp_group_manage_members_admin_item'` hook and that hook is really only useful with the `$groups_template` global.

Since the "Members" page manually constructs the group object instead of using the groups loop, the `$groups_template` global is never used.

View the following thread for the original problem:
https://github.com/boonebgorges/buddypress-group-email-subscription/issues/43

Feel free to reject this pull request as I plan on adding a similar hook to `'bp_group_manage_members_admin_item'` in BP 1.7's "Groups" admin page.