I agree with the above, start off slow.
Maybe try something like your girl can lock you / unlock you when she wants.
However if you 'want' to be unlocked (that isn't a safe word instant unlock) try asking her if you can earn your freedom. You may find you have enough fun out of that and your girl can come up with some interesting / fun ways for you to earn the key.
I'm sure that the fantasy of group of girls all knowing your secret and toying with you sounds great, however in real life it relies on both your girl being absolutely cool with that, plus all her friends being okay with that too. This maybe a hard sell for them all to be okay with.
Maybe you could ask your girl to do something like, if you want to earn your freedom. She can text 'Yes' or 'No' to a person of her choosing. She doesnt have to show you who she text, or explain to that person what it was for.
Unless she wants to.
Maybe she would text her friends and maybe they would be keeping you in chastity without them even knowing.
This way if your girl and her friends meet up, they might ask what the question was really for and then its up to your girl to expose your secret or not.