Pokémon Mystery Dungeon: Blue Rescue Team Tips

Recruiting Groudon & Rayquaza
First of all, make sure that you come with only one Pokemon of size 1. Rayquaza and Groudon are size 5, I believe, so you can only have one Pokemon. Also, their base recruitment rate is -10%. Therefore, I suggest coming at a high level, with the Friend Bow as a hold item (boosts recruitment chances by 10%). With the Friend Bow, you can recruit them at as low as Level 30, but the chances are minuscule. I would wait until around level 80+, so you have at least a 10% chance of recruitment.