A few weeks before the conference, create a FB group/discussion board/forum/mailing list and invite other people who'd like to abstain from alcohol at WhateverC

A few weeks before the conference, create a FB group/discussion board/forum/mailing list and invite other people who'd like to abstain from alcohol at WhateverConf after parties. If you're going to WhateverConf you probably learned about it on the web. There's no reason not to use the web to assemble people who want to have alcohol-free fun at WhateverConf parties.