There is no University policy regarding attire of guests. However, since the question is frequently asked, the webmaster for this site offers the following personal opinion:
Guests are encouraged to dress (1) respectfully for the occasion and (2) appropriately for the weather. Most guests tend to wear "business casual" (men: slacks and button-down shirt) although many "older" guests wear suits/ties or dresses. Suits are more common in the winter than in the summer. Shoes should be comfortable for walking between parking areas and the arena and for walking up and down steps in the arena seating area.
Although stage guests wear appropriate regalia because of their participation in the ceremonies (including uniforms for those in the armed forces), I don't recall having ever seen a guest of a graduate wearing a uniform except perhaps when their son was also being commissioned into the U.S. armed forces at a separate ceremony the same day.