How to recruit people into the blades?

I need to know how to recruit people to the blades (Also which ones are the best if any)

To recruit someone into the blades:

1. They need to be your current follower. If you currently have a follower that you don't wish to recruit, tell them that it's time to "Part Ways", and they will return to their home.

2. Travel with your potential recruit to see Delphine. Tell her that you'd like this person to be a recruit. Be aware that not all followers are eligible to join The Blades. Notable followers that are not Blade material are Serana (Goth chick from a very broken home), Talvas Fathryon (Aprentice to everyone's favorite 'shroom dweller), and anyone in the Dark Brotherhood. When accepted, the recruit's home will now be with The Blades and their default gear will be Blade gear. 3 recruits complete the ranks.

You may also wish to know that once a person becomes a Blade they will start using Blade gear, no matter how inappropriate to their fighting style. Blades use Heavy Armor, Bow, One Handed Weapon & a Shield. NPCs skilled in Light Armor may likely get an armor downgrade. If you use the list that Ben links to you may want to restrict yourself to "Warriors", who will generally find Blade gear to be an upgrade.

4 NPCs I like to use are:

• Vorstag, a mercenary in Markarth's SilverBlood Inn. (Max Level 40. The other 3 I mention Max out @ 30.)
• Uthgerd the Unbroken, the Female "Brawl Buddy" in White Run's Bannered Mare Inn.
• Ogol, an orc of "The Cursed Tribe". (Harder to acquire than the others. Daedra Heart & fighting a tougher than normal giant required to complete the quest.) His default gear is pathetic, so membership in the Blades gives him a serious upgrade.
• Cosnach, a drunk brawler in Markarth's SilverBlood Inn. ("Drunk voice" & sketchy attitude gets old quickly. Consider instead Ogol's respectful baritone.)

While there are other good choices, I find that these four are relatively easy to acquire, have the correct skill set & have a relatively high maximum experience level. Housecarls (except Valdimar) have the right skill set and max out @ L50, but many are awarded only after the purchase of a home. As Lydia precedes a home purchase, she makes a good candidate. The only other housecarls that precede home purchase are introduced in skyrim-hearthfire but, due to a lack of town guards near my rural estates, I rather my housecarls remain there to patrol my property.

Can the recruited blades help with the battle against Alduin ?

Your recruits into the Blades can, as a group, help you on Esbern's Dragon Hunt quests. This can be valuable for collecting Dragon Scales, Bones & Souls and, as each hunt will be at a word wall, your early hunts will also teach you some shouts. If that's not enough help, you can supplement your party with a non-Blade follower, a pet, a mount, and a summoned critter (or two).

After your 1st Hunt with the Blades, Esbern will accept one Dragon Bone & one Dragon Scale and give you in exchange a rather nice potion that will give you some permanent protection against the physical attacks of dragons. Esbern also provides a rather nice blessing that helps you slay dragons for a few days.

Blade Recruits will also, individually, still be available as followers.

BTW: If, in the course of the main quest line, you manage to piss off The Blades; All you really loose access to is Esbern's Blessings & Dragon Hunt Quests. (You did get his potion 1st. Right?) The recruits are still valid followers. Of course if you piss off the Grey Beards, you can no longer change which word you've most recently meditated upon and loose their help locating new word walls. (Hint: There are a finite number of word walls.) Once you've learned your shouts, this is really a choice between Esbern's help and Parthunax's help. (OK, sure, there's that whole moral issue too...)

"Battle against Alduin" Spoiler:

While members of the Blades can be of help as you prepare to face Alduin, (Dragon Souls, Gear made/tempered with dragon bits, Esbern's Potion & Blessing) you will be delivered to that encounter in a way that no follower, friend or organization will be able to assist. Summoned Entities & The Tongues of Old will be at your disposal. If still on speaking terms, get Esbern's Blessing before taking off from the Dragon Reach balcony.

Here is a complete list of followers that can be recruited into the Blades, as well as a rundown of the entire quest.

My understanding of it is that you can have up to 3 people join the Blades, most of them are followers (if not all). However, should a follower die, you can't recruit more (possible glitch), however you could possibly use console commands, depending on your platform.

From what I understand from your question here's a possible solution:

To recruit someone into the Blades, first you have to have a follower. Your current follower would be the one you're going to give.

As for the best recruits (really doesn't matter), you can check someone from the Dark Brotherhood.

As for your last question: Yes, they can help you with the battle against Alduin.