Photo/content release form for event photography and UGC (user generated content)?

We would like two things: 1) To take pictures at events of happy customers and partners. We want to use these pictures in marketing material with those peoples' faces on it. 2) We would like to have people share images on social media that we can later use on our own channels. How do I make sure I have the rights? What is the best technical way to do this? Can it be done through a website/app instead of printed paper at the event? Let me know what you do in these situations! Thanks.