Outlook calendar - see who has accepted a meeting without being the meeting organizer

It's by design. See here:

EDIT: Original link was broken and I can't find an updated version of that page. Here's another that says the same thing:

http://blogs.technet.com/b/ewan/archive/2012/04/20/tip-o-the-week-110-tracking-outlook-responses.aspx

If you're invited to someone else's meeting, you'll see options on how to respond, and you'll be able to look at the scheduling view to see who else is on the list, but you won't be able to see how they're responded to the invite (well not entirely). You may be able to see the details in the scheduling view (depending on whether the invited attendees have given you the permission to see their calendars).

If you didn't organise the meeting, you may be able to open the calendar of the organiser and still be able to see who responded and how.

I know this is an old thread about Outlook 2010, but it was the first search result addressing the same question for Outlook 2016. Informed guesses are as close as you can get if you're not the organizer and the calendars aren't shared.

If you open the occurrence you'll see the Scheduling Assistant button in the Meeting Occurrence tab.

It will show all the invitees and what you are allowed to see of their calendars. If their calendar is shared with you, you'll see their meetings. You'll just see colored and and patterned bars if their calendar is not shared. If there's no bar at the appointment time, you can be sure they have not accepted, but you can't know if they haven't responded or declined. Similarly, you can't know for sure if they're shown to be busy, if they accepted this appointment or some other event.

In Outlook 2010, click the Scheduling Assistant button on the Home tab; a list of invited people displays at left, along with their required/optional status. But as stated above, you cannot get the response status unless you are the organizer (that is where the scheduling tab comes into play).