if you buy friend areas you can do it
The land, Pokemon, and dungeons are extremely different.
Legendary Pokemon are usually found at the top of dungeons like rayquaza and kyogre. Most do not join you the first time you visit them.
Press the A button on the pokemon.
They don't open dungeons, they open locked rooms within dungeons. Certain unique items can be found, like the Silk items, and treasure items for Legendary pokemon.
35-40 dungeons i think
All can join your team
If you are referring to Pokemon Mystery Dungeon, just leave it in your bag, and you will find legendary Pokemon in some dungeons, like Lugia, Ho-Oh, and Darkrai.
2
You get them in dungeons
your mom and you
after credits
Some legendary Pokemon found in Pokemon Mystery Dungeon: Explorers of Time include Dialga, Regigigas, Azelf, Mesprit, and Uxie.