No special requirements actually, just carefully learn the project you want to participate in. You shall study the whitepaper, team, registration of the campaign etc. Good luck to you!
Most of the joiners in bounties they really don't care about the white paper or the team.
As long as there is a fund that is escrow or secured for the participants.
That is the most important thing for that bounty because some bounties are even offering higher rates but no assurance that they will pay.
escrow the funds ? Thats not really happen now this things only work if you join in a campaign which is paid in btc . If they paid token there is no securities for participants they will only distribute it after the campaign. Even the manager dont have control for the funds that will pay to participants .