You need to use Wardstone. (like other usable items).

Yeah I've tried that, I tried to put it in the quick use slots and it says wrong item type. I've also right clicked on it but that does nothing also.

I encountered a variation of this suppossed bug or glitch. Me and my friend are playing together, each controlling 3 characters and he took his 3 guys out and got teleported, but i took like half a minute to follow, by which time he had gotten teleported already and i was left stuck on the island.

The only solution we found was to reload an earlier save, fortunately we had saved after we cleared the entire cave, so we only had to walk to the exit, this time i went out first and my friend immediately followed and when we walked forward, we all got teleported.

My only suggestion is to try an earlier save game.

Sometimes I find I need to move the characters a short distance from the exit (once back at surface) before they teleport back. I have never had to "use" the runestone to get back

I have gotten this problem before. It caused me MUCH pain. I did not have a previous save before the ice cave to revert to, so my file was permanently stuck on the island. I had to restart the entire game.