The essence of the control that a self employer has over their business is that it is direct. They do not delegate their decisions to some trusted person who runs the business on their behalf; they do it themselves.
Preserving this aspect of self employment in a self employer joint venture requires that, to the greatest extent practical, every member’s control over the JV should be direct, rather than representative. Every member should vote directly on every significant decision that affects the business, rather than merely voting for a Director or CEO who represents them and makes decisions for them. Only trivial or repetitive decisions should be handed off to individual JV members to make on behalf of everyone else. The most important decisions about the JV, such as adding new members or expelling existing members, or entering into contracts with external parties, should probably require unanimous agreement of all members.
This obviously creates a lot of voting. In centuries past, coordinating a vote was difficult and time consuming, and this was a major driver for using hierarchical representative governance structures with multiple layers of management and poor to non-existent oversight of executives. Now that we all carry networked supercomputers in our pockets 24/7, coordinating a vote can be trivial, even in response to emergencies. This change is the main reason why self employer joint ventures are now a much more practical, responsive, and scalable business structure than they have been in the past.
Apart from being able to vote directly on business decisions, you need one more thing in order to exercise control directly: you need the ability to influence what decisions will be voted on. Every member of a self employer joint venture should be able to put forward proposals that their colleagues can vote on, that will be binding on the joint venture if the vote passes.