Hi @esedeerre
For complex fields, ACF stores the value in wp_postmeta as a serialized PHP array.
Hmm… I believe you should be able to update the user field using update_field(…) function. The code should look something like this:
$administradores[] = $new_user_id;
update_field( $field_key, $administradores, $post_id );
where $new_user_id is the ID of a new user and $administradores contains user IDs of the previously selected users.
