BuddyPress.org

Try <a class="button" href="<?php echo bp_root_domain() . '/' . BP_GROUPS_SLUG . '/create/' ?>"><?php _e( 'Create a Group', 'buddypress' ) ?></a>. Make sure you wrap it in if ( is_user_logged_in() ) as bp_root_domain() will be empty for non-logged-in users