First, you need to understand that the first time you recruit a particular NPC into your party - his/her abilities are determined according to your level.
Here's a list of NPCs and their weapon proficiencies, plus those that are bonus according to your level:

Aerie

Mace +
Quarter Staff +
Sling +

Club + (only if you are level 7 or higher)

Anomen

War Hammer +
Mace ++
Sling ++
Sword and shield style +

Quarter Staff + (only if you are level 10 or higher)

Cernd

Scimitar +
Dagger +
Quarter Staff +
Sling +

Single weapon style + (only if you are level 13 or higher)

I'm not sure, but it seems that if you're level 14 or higher he also gets a + in all these: Large Sword, Small Sword, Bow and Missile Weapons. (With the ToB expansion.)

Edwin

Dagger +
Quarter Staff +

Sling + (only if you are level 12 or higher)

Haer'Dalis

Short Sword ++
Dagger +
Dart +
Two weapon style ++

Long Sword + (only if you are level 13 or 14)

Long Sword ++ (only if you are level 15 or higher)

Imoen

Dagger +
Quarter Staff +
Short Bow +
Dart +

no bonus

Jaheira

Scimitar +
Club ++
Quarter Staff +
Sling +
Sword and shield style +

no bonus

Jan Jansen

Short Sword +
Dagger +
Quarter Staff +
Cross Bow +

no bonus

Keldorn

Two Handed Sword ++
Cross Bow ++

Long Sword ++ (only if you are not level 9, a bug perhaps?)

Bastard Sword + (only if you are level 9 or 10)

Bastard Sword ++ (only if you are level 12 or higher)

Korgan

Axe +++++

War Hammer + (only if you are level 8 or lower)

War Hammer ++ (only if you are level 9, 10 or 11)

War Hammer +++ (only if you are level 12 or higher)

Mazzy

Short Bow +++++

Short Sword + (only if you are level 8 or lower)

Short Sword +++ (only if you are level 9, 10 or 11)

Short Sword ++++ (only if you are level 12 or higher)

Minsc

Two Handed Sword ++
Mace ++
Long Bow ++
Two weapons style ++

Axe + (only if you are level 9, 10 or 11)

Axe ++ (only if you are level 12 or higher)

Nalia

Short Sword +
Dagger +
Quarter Staff +
Short Bow +

Dart + (only if you are level 13 or higher)

Valygar

Katana ++
Spear ++
Long Bow ++
Two weapon style ++

Dagger + (only if you are level 9, 10 or 11)

Dagger ++ (only if you are level 12 or higher)

Viconia

War Hammer +
Mace +
Sling +

no bonus

Yoshimo

Katana +
Dagger +
Short Bow +
Single weapon style +

Dart + (only if you are level 12 or higher)

Top