I remember a place to sign up to volunteer at the conference last year. Can't find it this year, but it might be part of the registration process - or it will be added closer to the conference. I know they have volunteers helping with seminars and other things.
They'll have a call for stewards and judges for the competition sometime in May probably. You need to be a BJCP judge to judge, but there are no qualifications to steward. They'll have a website where you can register yourself and indicate which sessions you're available for. The competition will be on Thursday.