Seeing Invite Acceptances

I run a bi-weekly women's group and need to see how many people attend each week. However, there's a handful of invitees who say they "Accept" my invitations, but come up as question marks as though they've not responded in my calendar. I think it has to do with them no "sending the response", but that seems like a stupid feature, when they're only trying to not clog by inbox by doing that. I'd also like to turn off the feature that requires Outlook to send an email every time someone responds, but that's less of a critical issue than not seeing who's coming. Thanks.