Pretty straightforward. If I equip my follower with set items, will he/she gain the set bonuses for multiple set pieces?
A follower cannot activate a set bonus for themselves yet. Check out the recent blue post here by Vaeflare.
As it stands currently, in order to receive the benefits of set bonuses, the set items need to be equipped on your main character. Set items equipped on Followers will not contribute to set bonuses.
We’re aware that this isn’t ideal, and we’re looking into some possibilities that would make using set items on Followers a more appealing choice in the future.