How can I get my party member "in range"?

Solution 1:

Party members in the back row cannot attack enemies, unless:

1. They're wielding a melee weapon that's labelled as a "Reach Weapon", e.g one of the spears those skellingtons drop.
2. They're a Rogue with 12 points in the Assassination skill, which lets them attack from the back row.
3. They're not wielding melee weapons, but are using ranged weapons (like rocks, bows & arrows, throwing knives, or spells)

So, in order to make sure all your party members can attack: put spear-shakers or rock-hurlers in the back row, and bare fists or melee weapons in the front.

If you had a character in the back row with a Reach Weapon who still couldn't reach an enemy, then something weird is going on; maybe the enemy was too far away in general? There might be some more mechanics involving Reach Weapons (like maybe a character in the front with a spear can attack at a greater distance than normal) that could be giving you the wrong impression about effective weapon ranges.

Solution 2:

At first only the 2 front (1, 2) characters can reach the enemies. That's how these games are made. But that also means that enemies in front of you can't hit the guys in the back.

The two back characters (3, 4) can only reach with missile weapons, spells, thrown items.

But the skeleton spears can reach from back there and the Rogue can learn a skill that makes him able to hit from the back row (require 12 in Assasination).