I wasn't quite sure how to answer this but I felt like I wanted to.
As someone who only writes only fiction, but with a professional background that means I could write either "advice" books or even reference books, I can appreciate the different quality of research that goes into me researching a story, compared to what I'd have to do if I wanted to advise someone on medication, for example.
I can't really tell if you're looking for a legal way to defend yourself from a lawsuit if some idiot like me suddenly decides to give you awful advice and a reader decides to sue you, or whether you just want a record of the people who talk to you consenting to have their answers / opinions included in your work so that they won't sue you if you do.
I like the spirit of the forums a lot. I like the idea of a number of people with different backgrounds sharing their experience in a casual and helpful way. I have no concerns that anyone who's ever offered me advice on woodcare, smoking teddy bears or army ranks will turn around and sue me later, and it's still my job to fact check anything I'm told.
I think your answer might be different depending on what you want.
If you want people to offer a little advice to help develop a work of fiction then I think I'd suggest you just go with the flow and ask your questions and see what advice you get.
If you want people's answers to be your book, i.e "I asked a hundred people called Craig what they thought about x, and Craig said...." then you've got a simple consent issue and you do need a record of people's agreement to you using their comments.
If you are actually writing a book offering medical advice to people and you're hoping to do your research through the forum, i.e. "Some guy on a forum claiming to be a doctor told me that eating cardboard is a perfectly good way to cure heart disease.", then I don't think anything you got me to sign would protect you from being partly responsible if you went ahead and printed that as fact. If you're wanting true experts to go on record and be responsible for any advice they give you, then I'd suggest you actually talk to a lawyer rather than make your own form up.
There's also truth in Robin's comments about what motivates people to respond. I usually get about four or five email surveys sent to me a year (I am a doctor) and the drug companies usually offer about fifty pounds worth of shopping vouchers for about half an hour of my time. I've never bothered to fill one in yet....but if someone asks a pointless question on the forum about how a psychiatrist might tell the difference between demonic possession and schizophrenia in a world where demons actually do exist....then try and stop me answering that one, and they don't even offer to bribe me.
You'll get more answers if you keep it casual, but if you're genuinely fearful of being sued, then it might be worth putting this question to a lawyer before you start.
Craig