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Yes, you can. You can write out the informal invitation letter to the US Embassy/consulate. The letter is informal, not mandatory, and simply helps the consular agent make a decision. However, it may be best that HE (the husband) fill it out since he is the one on the I-134. However, it is not required. The letter itself is not even required.
There is no need to send proof of identity/relationship - the letter can simply state who he/you is/are in relation. If you want to send in proof of documents, you can and it would not hurt but that is really considered going above and beyond.
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