Mateo had the (brilliant) idea to use the retainer and morale system as the base for a summoner class. His approach adjusts the loyalty rolls based on summoner level, monster HD, and other factors. This allows higher level summoners to control more powerful creatures (on average).
Here is another take on the same basic idea, but using the retainer system literally. That is, summoned monsters just occupy retainer slots. Reaction, negotiation, and loyalty rolls are made exactly as if the monster was encountered in the wild. Base rules are 3 LBB OD&D, but I suspect a similar approach would work with other systems. See pages 12 and 13 in Men & Magic and also this old post I wrote about OD&D loyalty and morale.
Monsters defeated in combat may be subdued and brought into service following the rules in Men & Magic. Now, we could pretty much just end there and have a relatively comprehensive system. Magic-users would need to find minions during adventures and convince them to serve or employ charms. However, I like the idea of magic-users being able to gate in creatures directly, and the only even remotely similar spell in Men & Magic is conjure elemental. The monster summoning spells introduced later are not to my taste. So it seems like at least one new rule is required, in the form of a spell that any magic-user could employ if learned:
Summon Monster, level 1 magic-user spell
In a puff of smoke, a monster appears. Use the dungeon random encounter tables, determining first dungeon level* then monster with dice. Establish reaction normally, adjusting for incentives (2d6: 5- hostile, 9+ friendly). Only one monster is summoned. Do not use the number appearing value.
I like the minions that Mateo crafted, but also find the idea of just leveraging everything in the Monster Manuals compelling.
Beyond the new spell, the core of the summoner’s art lies in the skillful application of other standard spells.
Summoned creatures cannot cross a properly prepared circle of protection (requires salt and casting the spell protection from evil). Creatures may be summoned into or outside of such a circle, at the summoner’s discretion.
Other spells that a summoner may want to master: charm person, charm monster.
The dispel magic spell may be used to banish summoned creatures. The “original spell caster level” is the higher of summoner level and monster HD.
The polymorph others spell may be used in conjunction with flesh to stone (using special techniques, they may be cast together, though both must be prepared separately) to transmute minions into figurines, holding them in stasis until recalled using dispel magic (no check required, given that you are dispelling your own enchantment). Such figurines are significant items, and creatures that resist are permitted a single saving throw versus the combined polymorph/stone effect.
Thus, higher level magic-users become better summoners because they are more likely to have all the other spells and be able to set up the summon, ward, compel “combo” more easily and reliably than a lower level conjurer.
In OD&D, I allow magic-users of any level to create scrolls of any spell possessed, even if it is too high level to prepare normally, at the cost of 100 GP per spell level and one downtime action. This rule is based on the scroll creation system in Holmes. It essentially allows ritual casting of higher level spells given time and sufficient GP. Thus, with proper access to spell texts and resources, even a low level magic-user could perform a summoning, but it would be a long, expensive, arduous undertaking, and still entail significant risk.
* Roll 1d6 for dungeon level if using The Underworld & Wilderness Adventures (page 10). The AD&D Monster Manual II probably has the best dungeon encounter tables (page 133), though, so I might use that instead, in which case roll 1d10 for dungeon level. The MMII provides the chance of randomly getting, for example, a duke of hell.