The lists don't add up the way you're describing, and @keystone named why. Buyer lists are specific. A cash buyer isn't a generic entity who wants any discounted property. They want a particular price band, a particular property type, sometimes a particular street grid. So ten members with twelve buyers each aren't 120 interchangeable buyers, they're 120 narrow appetites, and the value is that a contract that fits none of yours may fit exactly one of theirs.
The count matters less than you'd think. A member with six buyers who have each closed twice this year is worth more than a member with 300 names scraped off a list. The word "buyer list" gets used loosely for both, and the strict version is people who have actually bought and will answer your call.
Where your skepticism holds up: a network of ten people who all work the same three zip codes with the same buyer profile really is close to no network at all. They'll compete for the same sellers and refer deals nobody can place. So the question to ask about any group isn't how many members, it's how different they are from each other.
Something else sits underneath the sharing, which is that the members also pool what they know about compliance. Assignment rules, disclosure requirements, and whether marketing a property you don't own triggers a licensing issue all vary by state, and several states have tightened this. A group that talks about that catches problems earlier than a person working alone. Any specific question there is one for an attorney in your state.
