For every 2 GP spent in training, a retainer gains one XP. The PC in question also gains 2 XP, following standard treasure for XP rules. More than one retainer may be trained, though GP must still be spent for each independently. For example, training 2 retainers at once requires 4 GP to grant both retainers 1 XP. Fighter retainers trained by a fighter PC gain a +2 morale bonus during combat situations as long as the fighter PC is conscious, alive, and present.

The money so spent goes towards training facilities, archery ranges, laboratory equipment, paying local experts for advice and instruction, wooden swords, locks to disassemble, repairs, and so forth. Assuming that PCs own some sort of shelter, this paraphernalia will accumulate (lending the appearance of a training yard, dojo, laboratory, or shrine to the area in question).

I thought about requiring 4 GP per retainer XP gained for retainers of class different than the PC’s class, but I decided against it provisionally because 1) I don’t want players to become locked into playing only one class and 2) even though it “makes sense” that a character can train others of their own class more effectively, I think the same-class bonus is probably better represented by things like apprentice assistance (for magic-users) and morale bonuses (for fighters). Also, I like that the retainer advancement rules are not very long, and unambiguous.

I worried briefly that these new rules would not discourage taking along lots of zero level retainers, as they would not consume any XP unless trained. The traditional XP tax does not actually discourage taking along zero level retainers if they die, as XP is only divided between survivors. So even in the old rules, there must be some other reason to not use too many retainers. That cost is increased recruitment costs because of bad reputation, up to not being able to recruit new retainers at all. Also, there is the retainer limit based on charisma, which I may end up adjusting downward to discourage retainer armies (just because they are cumbersome to run during play).

For reference, the Moldvay Basic rules are (page B22):

DIVIDING XP Treasure is divided by the party, but the DM handles all the XP awards. At the end of an adventure, the DM totals the XP from all treasures recovered plus all monsters defeated and then divides the total by the number of surviving characters (both player characters and NPCs) in the party. EXAMPLE A party of 7 (5 player characters and 2 NPCs) goes on an adventure but only 6 come back alive. They killed monsters for a total of 800 XP and also collected 5800 gp in treasure, for a total of 6600 XP Each character receives 1100 XP at the end of the adventure (The DM may give each NPC 1/2 normal experience — 550 XP in this case — since the NPCs were “directed” and thus benefit less from the adventure).

This modification to the retainer XP progression rules was first proposed on Google Plus.