From my idealist perspective, reality consists of two fundamental kinds of things: experiencers and experiences.

There are emergent things as well: phenomenal worlds (vast collections of world-suggestive experiences).

So what you get is a picture of reality that looks something like this:

Experiencer #1 -------- phenomenal world #1

Experiencer #2 -------- phenomenal world #2

Experiencer #3 -------- phenomenal world #3

...

..

.

Experiencer #N -------- phenomenal world #N

Note how there is only one experiencer per phenomenal world. Each experiencer is shut up in their own private phenomenal world and isolated from every other experiencer. This means communication between experiencers is impossible because there is no way to share experiences. I call this “group solipsism”. Strictly speaking, it is not metaphysical solipsism since reality contains more than one mind, but it is still solipsistic because each experiencer is totally alone in their own world.

This is the reason why merely saying ‘other minds exist’ fails to resolve the problem of solipsism. What good is the existence of other minds if you’re unable to communicate with them? Unless some account of communication is given, then you're left with "group solipsism" – a position which is no more desirable than metaphysical solipsism.

So what is the way out of this problem? How is it possible for different experiencers to communicate with each other?

I do see a way out, but it seems really extravagant and speculative. The solution would be to say this: Although experiencer #1 is confined to perceiving phenomenal world 1, he is nonetheless capable of causally influencing phenomenal worlds #2, 3, …, N. So when experiencer #1 has the intention to raise his arm, this intention is transmitted to phenomenal worlds #2, 3, …, N and causes an experience of an arm to raise in each phenomenal world. And this is how communication becomes possible.

What’s strange about this solution is that I have no conscious knowledge of causing experiences in other people's phenomenal worlds. Shouldn’t I be consciously aware of the fact that I am causing other people’s experiences? It would seem so.

The existence of other minds is really weird. It is even weirder than solipsism, to be honest.