Add an activity item for a new custom post type item creation.

add-group-activity.php

<?php 

/**
 * Create an activity item to appear in a group. Fires once a post has been saved.
 *
 * @param int     $post_ID Post ID.
 * @param WP_Post $post    Post object.
 * @param bool    $update  Whether this is an existing post being updated or not.
 */
function add_activity_item_for_group( $post_id, $post_object, $update ) {
	if ( $update ) {
		return;
	}

	$bp = buddypress();

	$author_id = (int) $post_object->post_author;
	$user_link = bp_core_get_userlink( $author_id );

	$post_type_object = get_post_type_object( $post_object->post_type );
	$post_type_label = strtolower( $post_type_object->labels->singular_name );

	$group_id = 0; // Whatever your logic is here to find the right group ID.

	$type = $post_object->post_type . '_created';

	// Does an activity item already exist?
	$exists = bp_activity_get_activity_id( array(
		'component'         => $bp->groups->id,
		'type'              => $type,
		'item_id'           => $group_id,
		'secondary_item_id' => $post_id,
		) );

	if ( ! empty( $exists ) ) {
		continue;
	}

	// Create a post url that is relative to this group.
	$post_url = trailingslashit( ccgn_get_home_permalink( $group_id ) ) . $post_object->post_name;
	$post_link = sprintf( '<a href="%s">%s</a>', $post_url, $post_object->post_title );

	$group = groups_get_group( array( 'group_id' => $group_id ) );
	$group_url  = bp_get_group_permalink( $group );
	$group_link = '<a href="' . $group_url . '">' . $group->name . '</a>';

	// Only set hide_sitewide to false if this is a public group.
	if ( 'public' == bp_get_group_status( $group ) ) {
		$hide_sitewide = false;
	} else {
		$hide_sitewide = true;
	}

	$action = sprintf( __( '%1$s published the %2$s %3$s in the group %4$s', 'language_string' ), $user_link, $post_type_label, $post_link, $group_link );

	$args = array(
		'user_id'		=> $author_id,
		'action'		=> $action,
		'primary_link'	=> $post_link,
		'component'		=> $bp->groups->id,
		'type'			=> $type,
		'item_id'		=> $group_id, // Set to the group id
		'secondary_item_id'	=> $post_id, // The id of the post itself
		'recorded_time'		=> $post_object->post_date,
		'hide_sitewide'		=> $hide_sitewide,
		'content'			=> bp_create_excerpt( $post_object->post_content, 358 )
	);

	do_action( $post_object->post_type . '_before_activity_save', $args );

	$activity_id = bp_activity_add( apply_filters( $post_object->post_type . '_activity_args', $args, $post_id ) );
}
add_action( "save_post_{$post_type}", 'add_activity_item_for_group' );