App for communicating with guests

All of those things sound like they could be posted on your wedding website and that could be distributed through your save the dates. There are designated apps for sharing event photos like Eversnap or WedPics, but I think that social media and text message work just as well.

GroupMe, Kik, Facebook messenger

If you have Facebook you could always make a closed event and post updates on that.
Or a wedding website would be helpful that's for sure

Make sure it's on your wedding website. GroupMe, Kik, and FB are great... but there will always be people who won't be able to use it or refuse to. If you have guest who do not use facebook, make sure you have a way to get them the message still!

I would just put this information on your invites, on your website, and in your welcome bags in a pamphlet of some sorts. As a guest, I wouldn't want to receive several texts or emails regarding a wedding. Typically the save the date and invite are enough. I'm not even sure if an app like that exists, or if guests would even want to put in their phone or email? I think you'd just have to text or Facebook message each person.

Look into the Remind app, I think you can add people into a group/class without them having the app but they must text a code you give them to the reminder number. It's not too hard but might be complicated for those not up to date with technology.

My schools use it.

I think you could have a timeline, list of events and shared photos all on your wedding website. Or include a hashtag for easy access to finding them on your social media.