Beta exams are open to anyone that wants to pay the $125 fee. Microsoft does also give out free codes to people so they do not have to pay. Typically the free codes are given to people that have certifications already since they have "proven" themselves to have knowledge in an area. There are also times when Microsoft will email or send out codes to people that have subscribed to the certification email lists.
Ok, on to the questions...
One thing to remember is that there are alot of questions. There are actually more questions on the beta version of an exam then the normal version. This is because the nature of beta exams is to test out the various questions. This means that there are questions that are going to be thrown out. The challenge is that you never know which questions are getting thrown out. You could get all of the questions right that are thrown out, but you could still could fail the beta. The time for the exam is also longer. The last one that I took was scheduled for 4 hours. It did not take me that long to answer the questions, but there is also time to review and comment on the questions as well.
The number for the exam is 71-xxx instead of the normal exam numbers of 70-xxx. Not all testing facilities actually give beta exams. Check your local testing site to see what exams are available. Another thing is that the betas are time limited. There is usually about a month of time to take the exam.
Another big difference with beta exams is that you will not get your results back for 8-10 weeks instead of right away. This is tough when you are used to getting immediate feedback and results. It's a waiting game.
Beta exams are a mixed bag. You don't know exactly what is going to be tested, you will not get the results immediately, and you will spend a long time taking this exam. Good luck on taking beta exams if you want to try them out. I have both passed and failed some. They are tough, but rewarding.