My rule of thumb is: If they're traveling to the wedding, they can bring a guest. If they have a significant other, they can bring a guest. If they're in town, single, and they know a lot of people at the wedding, you're totally fine not giving them a guest.
People obviously adjust according to their budget, but the above is pretty safe.
One of my friends was having problems with one of our mutual friends and didn't really care if she came to her wedding. She invited the friend, but not her fiance...and they live in California. That effectively ended the friendship
