You cannot have "voluntary communism". The point of communism is to ban certain things -- e.g. make it illegal for private people to own capital. The entire definition requires that it be involuntary.

In capitalism, there are worker-owned companies living side by side with privately and publicly held firms. People can go off and create their own communist utopia and share their wealth. You cannot go off and create a capitalist utopian community in a communist system.

Because capitalism doesn't ban ownership structures (except for anti-trust against monopolies). It doesn't care about the outcome, it cares about the free choice. Economic liberalism -- for all its faults -- really is based on the principles of free association, enforcement of contracts, and voluntary trade. You can argue that the outcomes don't capture externalities, but you can't argue that they are involuntary.

Communism, OTOH, cares only about outcomes -- is this distribution of resources fair -- and does not care about free choice at all. This is because you have to forcefully take stuff from one person to give to another, and forcefully ban someone from running their own for-profit business. Of course communism couldn't do that perfectly -- there was always a black market of people who bought and sold for a profit. But they risked prison time for that, and sometimes were sent to Siberian labor camps.

Under capitalism you can create a contract in which people voluntarily pool resources and then distribute them according to some rule, say everyone gets a fixed payout irrespective of their position in the company. Therefore the economic system in which a group of people can choose to collectively own property is called "capitalism".

what happens when some people don't want to participate in your voluntary communism and insist on inventing new things and marketing them?

You'll note that membership in intentional communities is quite low.