Pretty much always, depending on number of players (the more players, the fewer NPCs). At low levels it was pretty much standard to hire a half-dozen or so 0-level men-at-arms, and/or to join forces with various NPC adventurers encountered in "town" or in the dungeons (such as the various characters seen hanging around in modules B2 and T1). The former are paid a salary and do not receive a cut of treasure or XP. The latter typically received a share of both, and were counted equally to PCs. So a typical low level party might be 4 PCs, 1 NPC adventurer, and 5 0-level men-at-arms. If they acquired 3K worth of XP from defeated monsters and 3K gp treasure from an adventure, the PCs and NPC would each receive 600 XP + 600 g.p. (=1200 XP total, assuming the treasure XP was rewarded at a 1:1 ratio), the men-at-arms wouldn't receive any XP (and wouldn't be factored into the division of XP) or any treasure beyond their salary (unless the PCs decided to share, in which case their own rewards would be reduced accordingly).
Starting typically around 3rd to 4th level PCs generally began to hire henchmen (i.e. low level NPC adventurers loyal to the PC) -- some players wouldn't acquire any (and might even be forbidden from doing so - rangers, monks, etc.) but most would tend to pick up 2 or 3. Not all henchmen would be used on every adventure (some would be left to guard the character's "base" or be sent on various offstage errands, etc.), but a typical adventuring party might consist of, say, 4 PCs and 4 henchmen (but might also be 2 PCs and 6 henchmen, or vice versa). Henchmen receive a share of treasure negotiated between them and their boss (usually, but not always, 1/2 share) and receive 1/2 the XP a PC would receive from monsters (with the other half being "lost"). So if a party of 4 PCs and 4 henchmen acquired 32K worth of XP from defeated monsters and 30K gp in treasure, each of the PCs would receive 4K XP from monsters + 5K gp (= 9K XP total, assuming the treasure XP were rewarded at a 1:1 ratio); each of the henchmen would receive 2K XP from monsters (half the normal PC award) and 2500 g.p. (half a share) (= 4500 XP total).
0-level Men-at-arms generally weren't taken on mid-high level adventures (since they couldn't accomplish anything except to get killed), but some characters maintained a body of them to guard their stronghold (and, of course, high-level characters who maintain a stronghold attract a body of "free" men-at-arms as followers). I never had any characters who attracted apprentices (i.e. actual classed characters) instead of 0-levels as followers (i.e. druids, magic-users, thieves, etc.), but if they did I assume they'd be treated the same as henchmen (only with better loyalty and no need to pay them).
