We've done mentoring before, both organically as Rebel mentioned, and with some staffers who formally took on the role and were assigned to people. It can be great when it works, but there were a few issues that I noticed.
One is that it's not always easy to match up growers with the same setup. I could become an mentor and certainly give someone general advice about the process, but if they are growing with a different soil mix and different nutrient combinations from those I've ever used, the amount of detailed help that can be provided is limited. I have often run into this when helping people due to the crazy amount of different grow products available around the world. So I end up researching nutes and soils that I've never seen and trying to do guesswork about feeding based on labels from the internet and gut instinct. Having the same lighting and nutrients goes a long way towards really being able to guide someone with specifics.
Another is simply time limitations, as well as the level of commitment from both parties. Anyone who has been here a while has seen how many noob journals dry up and the members disappear. When someone is already very busy, it can be frustrating to sink time and effort into helping someone who doesn't carry up their end of the bargain (been there, done that). The reality is that there will always be many more new growers who need help than there are veteran growers with hours in the day to help them all. That's why we have Live Time Help and we are trying to beef up our tutorials and guides. It's tough when you are working with someone and get busy with life, then they start to get spooked and next thing you know you're hammered with a dozen PMs. It's a tough situation because you know how upsetting it can be when you're new and a problem occurs with the grow, but at the same time most of us have families and jobs that can take our time unexpectedly.
One thing we are looking at to get around these issues is to move things to a classroom setting. Get a group of growers who agree to start on a certain date, and ideally have fairly similar growing setups. Then you have a group of teachers/mentors who work with everyone from beginning to end of the grow. That way you have a group of 'students' who can lean on each other and not feel like they're doing it all solo, and a group of leaders who can share the load rather than having the pressure of being the primary source of advice to a bunch of nervous new growers. It wouldn't be a perfect system, but it would be more communal and provide value to the entire site, unlike a couple of people sharing a lot of info via PMs.