Like any other pokemon. Keep defeating them, and their friend area is "Legendary Isle". I don't remember if this area comes with the three legendary birds, if not, keep defeating them and after a while one should ask and you will automatically receive access to the friend area. To get a better chance of recruiting Pokemon, give the leader a Friend Bow to hold.
i am not sure about if they are the leader but they can be a partner
In Blue Rescue Team, you can recruit Magikarp in the dungeon Waterfall Pond, 7F-11F.
All you need to do is get Articuno, Zapdos, and Moltress on your team. Then just talk to Alakazam and then you can go.
you do whatever you want, you can go back and recruit the legendary birds,(mt thunder, mt. blaze, and frosty forest) get the legendary dogs,( i forgot it's like someodd fieldsfor all three) or get Hoho(requires rainbow wing, you get parts of it from the legendary dogs)
you cannot recruit Pokemon for completing missions in blue rescue but u can recruit via missions in time and dark
I am absolutely positive of this answer, what you do is recruit Articuno, Zapdos and Moltres and then you talk to Alakazam in the Town Square. He will tell you to go to Legendary Island. The Legendary Birds will give you access to the Silver Trench.
You do not have to buy a place for the legendary birds. Once you recruit one of them, you get their friend area.
recruit ho oh
You can recruit Registeel in Pokemon Mystery Dungeon Blue Rescue Team but his recruit rate is 0.01 percent. Registeel can be found at the Buried Relic and his friend area is the Ancient Ruins.
I believe if you look at red rescue team it is the same.
You need to recruit one of the legendary birds. The island is automatically unlocked.
You get it when you recruit Ho-oh though I am not sure if you have to recruit it or just beat it