initiative
There are several ways to describe such a person. Titles like "volunteer" or "leader" could work depending on the circumstance. Also, if a person does something without being asked, they are usually described as initiative (as in "taking initiative," "being initiative," or "having initiative").
Initiator
No, it isn't. Initiative is a noun. We might say, "Building the new bridge is a great initiative." or "I see you took the initiative in introducing yourself to Mr. Smith."
An initiative taker is someone who takes initiative to door say something whether they want to or not. Ever heard of the phrase, "If I want/need something done, I might as well do it myself?" That's an initiative taker. An initiative taker would say something along the lines of, "This has to be done, it might as well be me that does it."
Individual initiative in anything, including economics, is when a single person decides to take some action, and does so.
You cannot "measure" initiative because it is a characteristic or quality of behaviour with no numerical basis. You can assess an individual's initiative by, for example, qualitatively gauging the efficiency and success of that person in solving a problem by it.
The root word for initiative is initiate. Initiate means to bring into being. Initiative means a person's readiness to act in an event.The root of initiative is -it-, meaning "goes." The latin word initium means "a going-in," hence " a beginning."
a person who organizes and manages any enterprise with initiative and risk.
An entrepreneur is a person who organizes and manages any enterprise, especially at business, usually with considerable initiative and risk.
A person who takes initiative, or perhaps one who is generous.
A person develops life skill and learns to take the initiative in childhood. Children learn to start taking care of themselves from showering to tying their own shoes.