According to the Grappling rules, once all the initial checks and movement are resolved, there’s no distinction between "grappler" and "graplee", at least that’s how I’ve always interpreted this:
Regardless of who started the grapple, while you’re grappling, you can perform only the following maneuvers.
That would mean you can’t just release a grappled opponent, because he’s grappling you too, you’d need to escape his grapple. There is one exception, you can release an opponent from a grapple if you Pin him first:
You can release a pinned opponent as a free action. If you do so, you’re no longer considered to be grappling that foe (and vice versa). You finish by moving into any unoccupied space adjacent to that in which you were grappling.
But I found recently that the Rules Compendium lists "Release grappled or pinned opponent" as a free action in the table Actions in Combat (p.8), but not in the Grappling section (p.60). And searching the internet I found a lot of people argue in favor of allowing this. Is this a typo, or else why isn’t this mentioned anywhere else?
If true, this would lead to exploits like a creature with multiple natural weapons (for example, 12 tentacles), low BAB, improved grab, and constrict, making several attacks and constrictions per round by releasing whoever they grappled immediately after doing constrict damage and attacking again.