B2 visa, for students who will be taking four ten hour classes in the US

[jhak0847]

We are a 501C3 US non-profit spiritual foundation, involved with providing training to our members. We have seven new members from Mexico. Our normal practice is to provide training in our spiritual practice in a US location. The classes are held over a weekend with a duration 10 hours each. there are four classes total with one held every two months. This would involve the new members making four trips to and from the US. For the purpose of assisting these members to obtain the necessary B2 visa, would we provide a letter to the consulate or the visa applicant? What are the important points that need to be in the letter? Thank you in advance for your assistance.