If a resource speaker or guest speaker is someone who makes usually formal public speeches; a trainer is someone who trains; how about someone who conducts seminar workshops?
Facilitator is sometimes used to describe someone who leads a seminar.
A facilitator is someone who helps a group of people understand their common objectives and assists them to plan to achieve them without taking a particular position in the discussion.
A person who conducts or presides over a seminar or group discussion may be called moderator:
one who presides over an assembly, meeting, or discussion.