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Apply a filter to “bp_get_group_description_excerpt” (located in bp-groups/bp-groups-templatetags.php).

The default is 20 words.

Try something like this in your theme’s functions.php:

function my_group_excerpt() {
global $groups_template;

$group = $groups_template->group;
return bp_create_excerpt( $group->description, 40 ); // change 40 to anything you want
}
add_filter( 'bp_get_group_description_excerpt', 'my_group_excerpt' );

Code hasn’t been tested.