Short answer - yes. Long answer - as long as bonus is applied at all, everyone in the same system will be getting the bonus, including the booster.

For squads the bonus will be applied as long as the booster is a member of a legal squad where legal squad means that there is a commander, at least one other pilot and no more pilots then commander's leadership*2 pilots total within the system. Some non-trivial examples: - Commander's leadership is 2. His squad consists of 4 pilots including the commander himself. One of those pilots is docked. He gets no bonuses (as if he needed them). Another pilot is in a different system. He gets no bonuses too. As long as commander and the last pilot are in the same system undocked, they get bonuses. - Commander's leadership is 2. His squad consists of 6 pilots - too much. If 5 of those, or all 6 are within same system, no bonus is applied. However should two pilots leave system or dock, the rest will get their bonuses. (at least that's how it worked some time ago. Not sure if it is possible to have a squad larger then your leadership*2). - Commander's leadership is 2 and he has a lot of bonus skills, but there are 5 other pilots and one of them has leadership 4, but no other leadership skills. It would be wise to make that other pilot a commander and set old commander as a booster.

In a squad the commander and the booster could be two different pilots. For wings and fleets only one pilot has to fill both roles.

One last note: of several similar leadership bonuses only highest one applies. If your squad booster has siege warfare at 4 and armored warfare at 2 while your wing commander has siege at 2 and armored at 4, you'll get 8% to both armor and shield (8% to armor for WCs armored 4 and 8% to shield for squad booster's siege 4). However, the WC himself will only get his own bonuses - 8% to armor and 4% to shield.