Apologies if this is a rehash, but:
Similar to the 'Open Invites' set-up but players enter their name and get put onto a 'mentor' list.
These players can only play one game at a time with an 'apprentice'. (Unless they put their name up twice, for example)
Upon placing their name on the 'mentor' list, the mentor stipulates the ranking range of potential apprentices. For example, an 1700 ranked player might accept an apprentice from those ranked 1300-1500.
The mentor also stipulates the time limits.
A mentor can remove his/her name at any time. Perhaps a 'remove name from list after current game' button ...
The mentor is under no obligation to play rematches with a particular apprentice.
During the game, the apprentice can send an in-game message such as, 'Why did you move there?!' etc - the mentor replies at the end of the game.
Once the game is finished, the mentor either PMs or sends as a final move in-game message some tips, pointers, advice etc. This can be of any length but say a minimum of 50 words or so ...
In theory any player, regardless of rank, can act as a mentor - a 1200 player could help an 800 player.
The whole thing is entirely voluntary - again, mentors can remove their names at any time.