How would you pick Tespia participants?

Variety in stats - pick players whose characters are least like others, either in base stats, outfitted armor, or bonuses. That's the kind of person who will find the unexpected glitch. Plus it gives a good cross-section of the player base for little effort.
Cumulative levels - to a threshold. Maybe 200+ levels on a single account, whether it be 1 main or 6 alts.
Useful bug tickets - This kinda assumes their ticket system is not utter pomegranate, and can find the tickets that were actually usable and passed along to the devs.