If the app is crashing, check if location services are turned on. Also check the privacy settings to see if permissions have been given to the WeddingMix app to access the camera roll. Go to settings>privacy>photos>WeddingMix and make sure it's turned on. Apple won't let apps operate with those two things turned off.
Another thing to try is restarting the phone. Some phones require a hard restart after changing privacy settings for them to actually take effect in the apps.
But, don't forget your guests can still capture media with their phones and then bring it in to the WeddingMix app. So, just let them know to take photos & videos with their phones outside of the app. We can help them import everything after your big day :)