Generally, a PC can have up to (PRE+Level)/4 (rounded down) henchmen at any given time. If they try to have more than that number, they spread themselves a bit too thin, and all of the various henchmen start taking penalties to their morale as a result. The means of contacting a potential henchman are numerous, and can range from hiring a town crier to bumping into the right person at a bar and buying them a drink. Getting the potential henchman to agree to follow the PC requires a 10+ on a PRE attribute check. A modified roll of 7-9 means that the potential henchman is uncertain, and that the PC can try again if they offer an extra incentive to help sweeten the deal. A result of 4-6 is a flat-out 'no', and a natural 3 or less means that the PC will suffer a -1 penalty on any potential future hires in the area, due to the potential employee trash-talking the PC behind his or her back.
The level of the henchmen can be determined randomly, if desired, by rolling 2d and adding the PC's level, then comparing to the following chart:
8- Zero-level 'man-at-arms' 9-12 1st level 13-16 2nd level 17-20 3rd level every +4 +1 level(this can and probably should be modifed by the size of the village/town/city that the hiring is being attempted at, but that's a post for another day)
Usually, a henchman won't follow someone who is of lesser level than their own, although there may be exceptions to this.
If the henchmen is 1st level or higher, then he or she is (roll a d6) on a 1-3 a Warrior, on a 4-5 some sort of Expert, and on a 6 a Magician (Paragons are almost never found as henchmen).
Henchmen usually start out with only 1d x the PRE in SP (if that) - the employer may need to pay for some of the henchman's basic equipment to get them to agree to hire on.
When following a PC/leader, the initiative rolls for henchmen cannot exceed that of their employer (they're looking to their leader to take the lead, after all). This goes for NPC leader-types as well - a goblin horde will not exceed the initiative score of their chieftain on any given round, for example.
Henchmen get full EP for facing threats and spending money on tithing/studying/carousing, half EP for exploration while in the service of an employer, and no EP for time spent playing at the gaming table (after all, they're NPC's).
Besides all the other benefits having henchman can provide, once per gaming session a PC may opt to deflect a threat/hazard/attack towards one of their henchmen instead, if such a thing can be explained in a reasonable manner (if the PC Drogo fails to disarm the poison needle trap on a treasure chest, it's not too likely that one of his henchmen is just going to happen to suffer the effects instead - however, if Drogo triggers a wire trap that causes a crossbow bolt to be fired from down the hallway, a nearby henchmen could instead be made to suffer for Drogo's lack of alertness).