Sunnis and Shias are considered to be Muslims, but then you have Qadiani who claim to be Sunnis and Alawis who claim to be Shia, but all within their respective schools have declared them kafir. How do we make sure that only real Muslims are answering or do you have to be Muslim to answer?
What is a real Muslim?
All Muslims think of themselves as real Muslims.
Another point, why should we not accept answers from non-Muslims if they are correct?
Filtering answers according to the person's beliefs (or lack of them) is highly discriminatory and bordering on ad hominem attacks.
Answers should be judged on their own individual merits, rather than on who the answerer is. The SE format should take care of that.
We had this very discussion on Christianity.SE when it first started beta, and we came to a simple conclusion (You can see the relevant meta discussion here) (adapted for Islam.SE):
Islam Stack Exchange is not a game show. We aren't here to vote people off the island. For the purpose of this site, please assume that the answer to "is X Muslim?" is always the same as "does X self-identify as Muslim?" It's the only way a group with such diverse cultures and beliefs can get along and do something productive.
I think the only way to run a successful site is to adopt the same policy here.
There's no reason to make sure that only "real Muslims" are answering; what matters is the answer, not the person it comes from. If anybody posts an answer that is wrong or outright harmful, it will be voted down (and possibly even deleted) by the community.
We don't want to divide the ummah, and I see no reason why those who follow Ahmad shouldn't be allowed to use the site, even if I (personally) disagree with a lot of their beliefs. Regardless of what others tell me, I won't treat anyone who claims to be Muslim as a kafir without clear evidences, nor do I recommend we as a community do so either.