Answer:
Our guests wished <u><em>they had seen our message about the time change of a meeting.</em></u>
Explanation:
In the given question, we have to change the form of the statement to make it start with "our guests wish...." To do that, we have to change the structure and also into an indirect form.
As the sentence requires us to start the sentence with "our guests wish...". the only way to continue with the sentence is to make it is to change the "disn't see" into 'wished', and then including the pronoun "they" to refer to the guests.
Thus, the final sentence will become
Our guests wished <u><em>they had seen our message about the time change of a meeting.</em></u>
