BuddyPress.org

in your theme folder /groups/index.php

find the <h3> at top replace with

<h3>

<?php _e( 'Groups Directory', 'buddypress' ) ?>

<?php if ( current_user_can( 'delete_others_posts' ) ) : //only admins and editors can see this ?> &nbsp;

<a class="button" href="<?php echo bp_get_root_domain() . '/' . BP_GROUPS_SLUG . '/create/' ?>">

<?php _e( 'Create a Group', 'buddypress' ) ?></a><?php endif; ?>

</h3>

This will show the button for only admins. You can still create groups if you go to the url.