AMA can be done in many different ways.
Asking a question directly to a well-known writer, athlete, scientist, etc might be a dream, but with Tribe this can be easily achieved.
In Tribe, any expert with a large number of followers around the world can have a room (groups), where audiences ask their questions directly from him/her. Members can up-vote each others’ questions they like, and at the end, the questions with more up-votes will be answered by the expert. This can be used for any person with many audiences around the world, before and during conferences, or in large classrooms.
The other way is that you can post picture with the host's name, date, and hour of the event, people leave their questions as a reply under the "discussion", the host will reply to the replies when the time is dues.
Or, you can ask users to post their questions in a specific group or topic page, depends on if you want everyone in the community to see the questions, then your host can have a live streaming to reply to the questions rather than writing the answers in the community. You can host the live stream as explained here: https://community.tribe.so/question/5d610682b136f0505c1df1b69