A question.

A command since it leads into a statement

I consider it an invitation.

Neither, although in its form it looks like a command. But the person doesn't really want you to guess. It's an introductory phrase meaning "I've got something surprising to tell you."

Guess what- question Guess! -command IMHO

it's a command. Ex: "Hey Bob, Guess What" "No"

The phrase "guess what" is an imperative statement in which the understood subject is "you." The word "guess" is a verb in this instance, and the word "what" is an interrogative word used as the direct object of the command phrase.

both

I guess it depends on how we say it , it could be both -- + up

A command.

I got marked off in third grade for writing it as a command instead of a question.

When someone says "guess what", I respond by saying "what?". Therefore it is a question

1) It is a question expressing a command. However, it could often be a rhetorical question, in the sense that you could have a guess without formulating it, and no answer is expected from you anyway, because the answer is given immediately by the asker. "A question may be either a linguistic expression used to make a request for information, or else the request itself made by such an expression. This information is provided with an answer. Questions are normally put or asked using interrogative sentences. But they can also be put by imperative sentences, which normally express commands: "Tell me what two plus two is"; conversely, some expressions, such as "Would you pass the butter?", have the grammatical form of questions but actually function as requests for action, not for answers; making them allofunctional. (A phrase such as this could, theoretically, also be viewed not merely as a request but as an observation of the other person's desire to comply with the request given.)" Source and further information: http://en.wikipedia.org/wiki/Question 2) Examples: "She looks like she has a _____! Guess what? She does! She looks like she likes to _____! Guess what? She does!" Source and further information: http://en.wikipedia.org/wiki/Guess_What%3F

It is usually a question. Imperative really. Command maybe. :)

question

Neither it's a figure of speech +2

It is not a question. It is a command. It does not request information like "What time is it?" or "What is the color of Tim's black horse?" do. It makes an order. It commands the person to make a guess.