Rule Question: Attacking an opponent who occupies the same square as a friend

[Tarantula Stew]

OK guys, the rule on this may be right under my nose, but I cannot seem to find it. We just started a new coastal campaign, and I decided to kick it off with one of my all-time faves, Sinister Secret of Saltmarsh. The party rogue managed to traverse the missing staircase treads leading to the attic, and eventually disturbed the stirges that were nesting there. The stirges scored a couple of hits, and I was reveling in the groans that were emanating from the other side of the DM screen. Where we got sidetracked, though, was when the 2nd character made his way into the attic to attack the stirges. Since stirges are tiny creatures, 2 of them were in the rogues space. What is the rule for attacking a foe that occupies the same space as a friend? Is there a penalty? Any chance of hitting the friend? Thanks!!!

[JHFizban]

If I were feeling nasty, I might give the stirges the +4 cover bonus and assume that they were flitting about and keeping their victim between themselves and its ally most of the time.

But under normal circumstances, I'd probably halve that to just +2. And if I were in a really good mood, no penalty, whack-a-mole!

[spalding]

From adjacent melee there is no penalty. Reach weapons act as ranged weapons for penalties in this case and ranged weapons would probably take penalties.

[spalding]

In the case that the stirge is attached we have the following too:

Quote:

Attach (Ex)

When a stirge hits with a touch attack, its barbed legs latch onto the target, anchoring it in place. An attached stirge is effectively grappling its prey. The stirge loses its Dexterity bonus to AC and has an AC of 12, but holds on with great tenacity and inserts its proboscis into the grappled target's flesh. A stirge has a +8 racial bonus to maintain its grapple on a foe once it is attached. An attached stirge can be struck with a weapon or grappled itself—if its prey manages to win a grapple check or Escape Artist check against it, the stirge is removed.

[spalding]

Quote:

Tiny, Diminutive, and Fine Creatures

Very small creatures take up less than 1 square of space. This means that more than one such creature can fit into a single square. A Tiny creature typically occupies a space only 2-1/2 feet across, so four can fit into a single square. 25 Diminutive creatures or 100 Fine creatures can fit into a single square. Creatures that take up less than 1 square of space typically have a natural reach of 0 feet, meaning they can't reach into adjacent squares. They must enter an opponent's square to attack in melee. This provokes an attack of opportunity from the opponent. You can attack into your own square if you need to, so you can attack such creatures normally. Since they have no natural reach, they do not threaten the squares around them. You can move past them without provoking attacks of opportunity. They also can't flank an enemy.

Quote:

To determine whether your target has cover from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target's square passes through a square or border that blocks line of effect or provides cover, or through a square occupied by a creature, the target has cover (+4 to AC).

When making a melee attack against an adjacent target, your target has cover if any line from any corner of your square to the target's square goes through a wall (including a low wall). When making a melee attack against a target that isn't adjacent to you (such as with a reach weapon), use the rules for determining cover from ranged attacks.

[Thorkull]

Personally I'd go with no penalty in melee, give them the firing into combat penalties for ranged attacks.

[Tarantula Stew]

Thanks, that makes sense.