Complex sentence: Participation is usually voluntary, so attendees who do not want their information disseminated can simply opt out by not putting their business card into the pool, though they can exchange business cards directly with other attendees as they please.
Split into simpler sentences: Participation is usually voluntary, so attendees who do not want their information disseminated are typically permitted to opt out by not putting their business card into the pool. Non-participating attendees are typically permitted to exchange business cards directly with other attendees as they please.