How exactly does holy symbol work now (Post bb)

I think 2 people is enough, but the HS'er has to be within leech level (so 5 levels under the monster or the attacker). If HS'er isn't, you need some other party member that is.

Solo HS only gives 10% extra.
Party HS gives 50% total, but since the HS'er is leeching you only end up with about 20% more exp than you'd get solo with no HS.