I have been responsible for the design of many technical conferences; the first answer is that Flynn would be a draw resulting from his stature and knowledge.
Because General Flynn was fresh out from the government, there are constraints on what he could do and speak about. The General knew very well that participating in a Russian-sponsored event would be "edgy" to say the least.
"Flynn was paid $45,000 by RT in 2015 to speak at the group's 10th-anniversary gala. The two other companies – Kaspersky Government Security Solutions, a U.S. subsidiary of Russian cyber security firm Kaspersky Labs, and Russia-based Volga-Dnepr Airlines – paid him more than $5,000, according to the documents."
General Flynn, like most former federal government officials, engages in speaking and consulting after their government careers. It is a part of the "revolving door" paradigm. That is one thing about Donald Trump with which I agree. He wants to end the cycle of former Congressional representatives joining lobby firms. Add to that, former military brass.
The subject is Michael Flynn wanted the money. Maybe he had the idea that by meeting and knowing possible "enemy contacts" in the cyber war would somehow strengthen his credentials.
This circumstance highlights that the primary motivation is money. It isn't patriotic. That is the major problem in the American political system today.
Image from Naples Herald