In order to define religion you must first define several other things. Is Taoism or Confucianism a religion? I would argue not. This is because it is so disorganized and so unclear on its values that it doesn't even have a clear message. Once we know that then we know religion must have a clear set of values.
Next we must define how organized it must be. Should it have churches or places of worship? Or should it just have a group of people who are organized and believe the same thing. This is a much harder question, but I suppose that it would need to be organized enough so that they can get their message out to the people. They need a way to convey their message.
That last thing I want to cover is how large the group should be. The answer is that there is no set limit or minimum number of people there need to be in order for it to be a religion.
So in conclusion I would say that religion is a group organized enough to get their beliefs across with a clear set of values, but has no real limitations on how many people have to participate. Of course, this is just my humble opinion.