On the Check it page, Discover Agents names at least two customer groups to test the idea with: real, specific groups, never "everyone".
One side or many?
First it works out whether the market has one side, two sides, or several. That matters because when there's more than one side, you have to win over each one. A marketplace, for example, isn't real until both the people using it and the people supplying it want it, so you'll see a group for each side.
Each group is different
Every group gets its own profile (what they're trying to get done, what's frustrating them, what they'd love) behind a Customer profile popup, plus a one-line "why test with them". Don't expect the profiles to match. If two groups wanted exactly the same thing, they wouldn't be separate groups.
Use these as your shortlist of people to actually talk to when you run your tests.