Change BP Docs capabilities so that group admins and mods can read and edit any doc associated with the group.

bp-docs-group-admins-can-read-and-edit.php

<?php
/**
 * Allow group admins and mods to read and edit any BP Doc 
 * associated with a group they moderate.
 * Note: this will allow direct access at `/docs/doc-title/` but
 * the docs will not be listed in the group docs directory if the 
 * access level "should" prevent the user from accessing it. 
 * So it's of limited usefulness.
 *
 * @param string[] $caps    Primitive capabilities required of the user.
 * @param string   $cap     Capability being checked.
 * @param int      $user_id The user ID.
 * @param array    $args    Adds context to the capability check, typically
 *                          starting with an object ID.
 */
function dc_custom_groups_map_meta_caps( $caps, $cap, $user_id, $args ) {
	switch ( $cap ) {
		case 'bp_docs_read' :
		case 'bp_docs_edit' :
		case 'bp_docs_view_history' :
		case 'bp_docs_manage' :
		case 'bp_docs_read_comments' :
		case 'bp_docs_post_comments' :
			$doc = bp_docs_get_doc_for_caps( $args );

			if ( empty( $doc ) ) {
				break;
			}

			$group_id = bp_docs_get_associated_group_id( $doc->ID, $doc );

			// If not associated with a group, nothing to do here
			if ( ! $group_id ) {
				break;
			}

			if ( groups_is_user_admin( $user_id, $group_id ) || groups_is_user_mod( $user_id, $group_id ) ) {
				$caps = array( 'exist' );
			}
	}

	return $caps;
}
add_filter( 'bp_docs_map_meta_caps', 'dc_custom_groups_map_meta_caps', 15, 4 );