I don't think that everyone gets invited, but I could be wrong... I'm not sure if they've made this automated or not but it used to be that people were hand picked by their community manager (Kimadhd is her name here I believe).
Payment can be a bit on the confusing side, once you are a paid guide/shopping assistant you get paid for every question you answer that is in your area of expertise as long as you are the first guide to answer... You get 35¢ if you answer and meet those qualifications, you get 65¢ if you get a thumbs up on your answer from the person who asked the question, and then if you get best answer you get $1 on top of your other earnings... I'm not sure if the best answer only applies to tip questions or not though.
I'm sure Kim will pop up with an answer for you soon, I just figured a quick overview might help if you're hesitating to respond
I think it's a great little egg and Kim is amazing, she really goes out of her way to take care of us even though it can be a frustrating task for her at times.