I'd use this feature.

As someone who can't play several hours per day every day, utilizing and maximizing those "bonuses" is sometimes the only way I cap my tomes each week. The concept would work well, assuming there is only 1 new player per group of 4, or 2 per group of 8, it should be easy for veteran players to either teach the new ones how to run the dungeon/trial, or carry them through if absolutely necessary. Certain players would avoid that roulette, which is fine, because many veteran players don't mind teaching others, especially with a bonus involved.