You are correct. It is better than nothing. But I am going to give you some information that I think may help on the other parts that would give her a better chance of approval. First, however, the letter of invitation should be on event organization letterhead, should specify the dates of the event and location, should clearly state what the event is for, why they want to invite her, and how perhaps she will be an integral part of the event.
Along with the letter, the organization should include anything they can as far as literature, advertisements for the event, perhaps a copy of the lease agreement for the venue, etc., just to legitimize the event and organization.
Now, as to the additional information that I was going to give you, I think it is important for you to click on the link below and read the article that it takes you to very carefully. I think it can help you structure a good case.
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