Is sign up one word or two words?

Answer 1

"Sign up" and "sign-up" (also spelled "signup") mean two different things. "Sign up" is a verb, an action. "Sign-up" is a noun, a thing; sometimes it is also an adjective, which describes a noun. For example:

"I used the signup link to sign up."

I'll throw in some subjective information and say that the fact that 98% of people on the internet use these words wrongly bugs the crap out of me. They need to hire a proofreader who knows their English. I volunteer!