Show teaser content to non members for the plugin Groups. Add the code to your theme's functions.php

gistfile1.php

// Show title, excerpt and thumbnail in archives
add_action('pre_get_posts', 'my_pre_get_posts');
function my_pre_get_posts() {

	// If the user is looking at the front page
	if ( is_archive() || is_category() || is_front_page() ) {

		// Remove the Groups_Post_Access filters
		remove_filter( 'posts_where', array( 'Groups_Post_Access', 'posts_where' ), 10, 2 );
		remove_filter( 'the_posts', array( 'Groups_Post_Access', 'the_posts' ), 1, 2 );
		remove_filter( 'get_the_excerpt', array( 'Groups_Post_Access', 'get_the_excerpt' ), 1 );
		remove_filter( 'the_content', array( 'Groups_Post_Access', 'the_content' ), 1 );
	}
}