Well, there are certain Pokemon (but you need the GUide Book, and it costs 19.95) that learn the IQ skill 'Fast Friend' if you feed that certain Pokemon enough Gummis (that it likes) and it learns that skill in 5.5 stars. Another is to get the items Golden Mask or Friend Bow. There is also, but you have to have the Secret Rank, 7 challenging missions where you pick up 7 unique friend items to recruit Pokemon like ice, fire, electric, etc and the challenging part is your lead Pokemon must be at least leveled 75 plus and have Discharge/Blizzard, a move that hits multiple Pokemon in the room. Bottomless Sea, one of the challenging missions, has 49 floors, World Abyss has 24 floors, the dungeons vary. You then take on the 'boss Pokemon' of that dungeon. If your' lucky, you can recruit that boss/at the same time, get the 'friend item (aqua monica)'. The key is though you got to defeat the boss.
Your lead Pokemon, if you get it's level high, Pokemon will join your team, it just takes patience and going back to the dungeons over and over until you recruit that Pokemon.
Another way to recruit Pokemon is while you go to the Spinda's bar to give your Pokemon gummis, sometimes Pokemon will drop by complimenting your taste in how you sample the drink and then ask to join your team. Only certain Pokemon drop in though, no legends, they're not evolved, and they're leveled 1. Keep an eye on the Pokemon that want to join your team though for sometimes, you may wind up with 4 of the same Pokemon as I discovered in my chamber.
Hope all this helps.