How did you get your first expert network invitation?

I've been seeing more people mention expert networks lately, especially consultants and people who've worked in pretty specialized industries. From what I understand, companies sometimes pay for short calls with people who have firsthand experience in a particular field, which honestly sounds interesting. What I'm curious about is how people actually get their first invitation. Do these networks usually reach out through LinkedIn, referrals, or is it worth creating a profile on one of the platforms yourself? If you've done expert calls before, what was your first experience like? I'm less interested in the payment and more curious about how the screening process works and whether you felt like your industry experience was enough, even if you weren't in a senior executive role.