I hope that my colleagues will expand on this.

That's an over-elaborate explanation. In that sentence, "will" simply expresses futurity. "I hope my colleagues are going to expand on this."--means the same thing.

I asked a question in a forum one of the teachers answered me and in the end of his answer he wrote that sentence. Do you think it isn't a invitation?

This is what I meant by saying it's over-elaborate. In that sentence, "will" expresses futurity--invitation is an unnecessary grammatical category. I guess if one is a grammarian, that's how one justifies one's pay check.

There are only four types of sentences in English. Declaratory sentences state something, they are statements. Interrogatory sentences ask a question, or more than one question. Imperative sentences give a comman, they tell someone to do something. Exclamatory sentences express surprise, dismay, excitement, anger--they are forceful expressions of emotion.

Any attempt to categorize sentences any more finitely is an exercise in pedantic futility. If your teacher believes in sentences which are "invitations" or words which express invitation, then learn it long enough to keep your teacher happy, and then forget about it.

Thank you so much.
What do you think about the sentence below? in the following reference it is said that 'will' is used for certainty in the future. but I think it is a question and there is not any certainty.
[talking to a child]
Will you be 5 in September?
Reference: https://dictionary.cambridge.org/grammar/british-grammar/modals-and-modality/will.

"Will" in that sentence expresses futurity. Yes, it is an interrogatory sentence, it asks a question.

I read about 'invitation and requests',... in the following link:
https://dictionary.cambridge.org/grammar/british-grammar/modals-and-modality/will

Don't you believe about certainty?

Click here to read the brief Wikipedia article on communicative competence.. I am assuming that you are learning English for pragmatic reasons--to use in employment or in an educational setting. If your goal is academic recognition, to become a linguist, or worse yet, a philosopher of language . . . don't call me, I'll call you.

I am beginner learner I never want to be philosopher or linguistic I only want to learn English that I can be able read books easily, can be able to understand English speekers, movies....
but the main usages of 'will' is very difficult because of 'be going to' I never can recognize which one I can use.

I'm not sure I can help you with that--but my advice is to read plays, such as the plays of Neil Simon. When I wanted to learn how to speak French ( I had already taught myself to read it), I read plays. It can give you a sense of the rhythm of a language, and the common expressions used in that language. Some things are arcane--they are neither logical nor intuitive, you just have to learn them. Plays are a good way to learn those things because in plays, the characters are speaking, usually, in an ordinary manner. Do a web search for Neil Simon, and then write down some of the plays listed in his biography. Then you can search the web for copies to read, or look for them at the library, if there is a library near you with English language books.

Mr. Simon's plays are in idiomatic English as spoken in the United States. I would not be confident in recommending plays which employ British usages.

Thank you so much.
What do you mean with "plays"? Do you mean "musics"?

Plays are productions presented on a stage with actors. Shakespeare wrote plays, such as Hamlet, King Lear, Julius Caesar and so forth. Here is an image of a stage play:



The plays of Neil Simon are comedies, and they are about ordinary people, and everyday life. That is why they might be useful for you. Some good plays which he wrote (and which were made into movies) are Sweet Charity, Barefoot in the Park, The Odd Couple and The Sunshine Boys.

Thanks a lot Senata. I appreciate your useful help.

You're welcome. Good luck in your studies.

Asking questions of those who know little of the English language and how it works is a recipe for disaster for EFLs. In another thread these same people will rail on and on about how English has nuance.

OP: I think 'will' in the sentence bellow announced 'invitation', am I right?
I hope that my colleagues will expand on this.

Yes, you are right and Setanta is mistaken. To suggest that it only is an expression of futurity is mere gobbledygook. Totally unhelpful gobbledygook.

Note the seventh definition.

McMillan:
modal verb
▸used for talking about future actions or events more...
▸used for saying what you expect to happen in the future more...
▸used for saying what is planned or arranged for a future occasion more...
▸used for showing that you are fairly certain that something is true more...
▸used for saying that you are willing to do something or that you intend to do it more...
▸used for asking someone to do something, especially when you are annoyed more...
▸used for making a polite offer or invitation more...
▸used for saying whether something is possible more...
▸used for ordering that something must be done more...
▸used for saying what always happens in certain situations more...
▸used for saying that someone has a habit of doing something, especially when you do not approve of what they are doing more...

...command.....