Sign up is a verbal phrase. It means to commit or agree to participate in an endeavor such as a class, the military, or a business deal. It is the act of putting one’s name to a contract or list. The past tense of sign up is signed up, the indicative is signing up.
Signup is derived from the phrase sign up. As a noun it is a place, sheet of paper or other device where one might sign to agree to participate. As an adjective, signup modifies the place, sheet of paper or other device where one may sign up.
Those who had been in the 1984 floods or the Canterbury earthquakes knew what effect a natural disaster had on a city and should sign up to protect themselves, he said. (Southland Times)
The other 49 countries that signed up — seven others are still waiting for approval from their domestic legislatures — included Germany, the U.K. and South Korea. (International Business Times)
Thirty-seven mushers signed up in person on Saturday at the Iditarod headquarters in Wasilla and 25 others entered by mail. (Fairbanks Daily News-Miner)
Currently, the signup is available only in Europe, with no time frame announced for how long the registration link will be available. (International Business Times)
Users of the latest iOS 8.4 public beta are encountering a signup prompt for Apple Music a full two weeks before the service is scheduled to go live, according to tips received by AppleInsider. (AppleInsider)