You can't end your movement in the same square as an ally, but you can move through their square. There are exceptions for a helpless creature and creatures of different size categories.
You can move through an unoccupied square without difficulty in most circumstances. Difficult terrain and a number of spell effects might hamper your movement through open spaces.
Friend: You can move through a square occupied by a friendly character, unless you are charging. When you move through a square occupied by a friendly character, that character doesn't provide you with cover.
Opponent: You can't move through a square occupied by an opponent unless the opponent is helpless. You can move through a square occupied by a helpless opponent without penalty. Some creatures, particularly very large ones, may present an obstacle even when helpless. In such cases, each square you move through counts as 2 squares.
Ending Your Movement: You can't end your movement in the same square as another creature unless it is helpless.
Overrun: During your movement, you can attempt to move through a square occupied by an opponent (see Overrun).
Tumbling: A trained character can attempt to use Acrobatics to move through a square occupied by an opponent (see the Acrobatics skill).
Very Small Creature: A Fine, Diminutive, or Tiny creature can move into or through an occupied square. The creature provokes attacks of opportunity when doing so.
Square Occupied by Creature Three Sizes Larger or Smaller: Any creature can move through a square occupied by a creature three size categories larger than itself.
A big creature can move through a square occupied by a creature three size categories smaller than it is. Creatures moving through squares occupied by other creatures provoke attacks of opportunity from those creatures.
Designated Exceptions: Some creatures break the above rules. A creature that completely fills the squares it occupies cannot be moved past, even with the Acrobatics skill or similar special abilities.
Oh yeah, and if you do accidentally end on an occupied space, the rule for that is a little lower on that same page of the PRD:
Also relevant is the section on Moving Through a Square.
A Fine, Diminutive, or Tiny creature can move into or through an occupied square. The creature provokes attacks of opportunity when doing so.
So if your party is composed of Gnomish and Halfling spellcasters who keep themselves miniaturized through reduce person, you can have them four to a square (as per Big and Little Creatures in Combat).