NPCs with their own interests and goals will join your adventuring party if you allow them to. There are twenty-five such NPCs scattered around Baldur's Gate. You will have complete control of their actions, although occassionally they may disagree and not listen. Be sure to find an effective mix of characters as some NPCs like and complement each other, while others will be antipathetic and try to verbally or physically spar. Your alignment and reputation are also important factors in determining whether your party will stay together happily.
The party may have a maximum of six characters permitting many combinations and much variety. To ease controlling each member, scripts can be chosen to set the default behaviour of the NPCs.