I have a player (sphere) and 4 enemies (capsules). All enemies have a character controller added. They also all have a child GameObject, which has a RigidBody (no mass, no gravity) and a Box Collider attached. The Box Collider is large enough so you can see the sphere model inside of the box, even with a little space in between.
The player also has a character controller, but no RigidBody. It does have a sphere attached with a mesh collider.
All of the enemies have the IsTrigger box checked. Why don't they call the OnTriggerEnter() function? Note that when enemies collide, OnTriggerEnter() is called. But it doesn't work on the player.