How to see users who opened a message?
You can see those users in the Audiences section, following these steps:
1. Create a new audience, and add filter "Total Occurrences" of event.
2. For event name, search for the message name and the event Open. If it's not yet available, follow this article on how to search by event ID: How to target users by Campaign or AB test events, that are not yet available in the autocomplete drop-down list?
3. Select "is greater than 0" for the event occurrences.
4. Click on Explore users to see them. From there, you can download the list of user IDs if necessary.