Hi!
The person/organization who offers the bounty is the one who set up the rules, usually bounties require you to be atleast a full member( They want to prevent people to join from multi accounts).
About the followers thing, the more followers you have is the higher reward you will receive for your work, I would suggest you to build your social media account with your bitcointalk account, the higher level you get it to be, the more you would earn from bounties.
Good luck!
Thanks!