I am in the case of studying the idea of creating a online forum for doctors, How can I get information about this groups, and their opinions?
A focus group or send out a survey.
You can ask people's opinion and feedback about similar product they use
How many opinions are you hoping to solicit initially? If the answer is, "only a few", then get connected to a couple of physicians through friends or relatives, then offer to buy them lunch for 30 minutes of their time. If you're hoping for a much large sample of responses, then join an existing online forum for doctors (https://blog.capterra.com/9-doctor-social-networking-sites-every-physician-should-know-about/), then get networking.
Surveys are really good, but you need to be careful how you ask the questions.
Be sure to structure the questions so that they don't bias the responses to justify any preconceived notions you might have about the viability and usefulness of the idea. Otherwise, you just create an echo chamber.
Focus more on asking questions about the problem you are trying to solve versus the product idea.
Prepare questionnaires on the pain points you address , speak to user group and get your answers
I really like and highly recommend the book Running Lean by Ash Maurya. He goes into a lot of detail about how to conduct interviews with potential customers, including something resembling a script to follow.
Doctor community is not very much available online in many countries. So, best is to prepare a very short but concrete survey and get it filled by doctors only ( End users ).
Also, ask one question: " What if this survey was online? would they participate? " - note this answer very carefully ( If possible read expression), that will be an entry point for your product " Online forum for Doctors".
Let me know if you need any help in creating survey.