How to change size of group thumbnails?

Open functions.php and change size here:

define( 'BP_AVATAR_THUMB_WIDTH', 50 );
define( 'BP_AVATAR_THUMB_HEIGHT', 50 );
define( 'BP_AVATAR_FULL_WIDTH', '215');
define( 'BP_AVATAR_FULL_HEIGHT', '135' );

Also open style.css and change size of thumpnails here:

.group-box { ... width:215px; ... height:245px; ... }
a.group-box-image { width:215px; height:135px; ... } 
.single-group-box-image-container { width:215px; height:135px; ... }

Now open header.php and change crop tool ratio here:

aspectRatio: 1.59