It's always active but you can't see the radius. During combat you'll see a buff icon above the character's portrait if they're affected by leadership and it disappears when they move out of range. If your leadership character has high charisma the range is fairly large.

During combat you can actually see it - at least you could in the original version. Put your mouse over the flag-like icon above your characters' portraits, and you should see a white field on the battlefield, centred around your person with high leadership/charisma. Also worth to point out that you don't just get better control over your NPCs with high leadership (less/no chance for them to go rogue), but you get bonuses to hit chances. That can be quite useful in the beginning when you don't have high skill levels on the various weapons.

I have the impression that the control against the rouge actions don't has a range, as long your character with the leadership is alive will control the other 3 NPC.
However, the radius for the +Critical chance is determined by the Charisma attribute .

It's amongst the skills I've paid no attention to (along with animal whisperer, etc), so, out of curiosity : how does it work with multiple chars having charisma/leadership skills ? Each one has his/her own sphere of influence, or does it only register the highest sphere as "the" group leader - in which case how does it deal with ties ?

There is a leadership radius line that pops up in combat, I`m sure I`ve seen it. Anyway, you don`t need to activate anything, just check your Follower`s Rogue chance it willget less and less as your Leadership grows. My main character is Leadership 6. The small info pop-ups say that it increases the chances of Followers following orders by 30%. However, when I check the character chance to go Rogue, it tells me 0%, which seems to be the case as no one in my team goes rogue any more unless my character is killed. You can often tell when it`s working as sometimes in combat a writing pops up saying, `Angie was calmed by X`s Leadership.` She was about to go rogue, but your Leadership presence stopped her.

A good leadership score increases your team`s to-hit chance as long as they remain in the radius (which is quite big by level 6). My team`s to-hit chance is now increased by 12%. It really helps. Only my lead character has leadership so I have no idea how multiple leadership works. As for Charisma, combined charisma does help towards the chances of a Follower joining your group. Even the little blurb says that no one wants to join a ragtag smelly team. Ironically, I have just employed a Tramp with a charisma of 1, but the rest of our Charisma (and my Leadership) helps to offset the hit! Please, it`s Rogue, not rouge, rouge is a reddish colour and sounds completely different. Always think the `g` is in the middle of the word. Sorry, I just had to say as you always spell it wrong, exactly like a friend of mine! :)

Didn't catch my error, many thanks man!

The radius is governed by your charisma as opposed to skill level, isn't it? And yeah, it's an extremely handy skill - my team leader is not at all combat oriented, but increased chance to hit is major, and there's a perk which slightly decreases enemy chance to hit as well.

That's how I like to play too. 10 Charisma on the leadership figure, and add Leadership skill points as he levels up, trying to get it high enough early-ish on so that no NPCs will go rogue. Some have a higher chance of that occurring than others.

High charisma also helps to gather more XP, and by the end of the game it can be a pretty big swing between a charisma 10 and cha 1 character.

I actually somehow missed that and the rest of my party is at CHA 1. I'm sure that won't bite me in the ass in the end :-P I'm actually thinking of just increasing CHA as I get attribute points, the rest of their stats are quite well rounded (obviously)

I also created a strong leader type who isn't very good at combat (though that last trait was not by design ;) ). The importance of this character is made clear on the occasions that she's knocked out, in which case pretty much all my NPCs instantly go rogue.

a little off-topic, but that let you get the mutant girl ? I understand that for recruiting her (and other) you need a minimum of charisma (sum of all the members)

I just got permission to access Ranger Citadel and since I never got too far in the game, I don't even know what is it that you're talking about :-P

Erh... so, in this moment you can read this (spoiler) or not: Some recruits requires that the sum of the party's charisma be a minimum. Let say, this is the bad thing about using Cha as a dump stat

On the other hand, it's a benefit to having a character with 10 charisma. The 6 other people will then usually be enough to recruit some characters, unless they require very high combined charisma. With all four at 1 charisma, you're basically out of luck.