Your group's organizer changed it for some reason. You can contact them to ask them why that happened.
Pay By Group can't charge you more than you initially agreed to be charged without your permission, so if you want to stay in the group, you'll need to accept the cost increase.
There are three things that could have happened:
1) The organizer increased the total price of your group
2) The organizer decreased the minimum amount of people to split this cost with
3) The organizer increased the "minimum commitment amount" on the groups where you need to specify how much you're paying in. The new "minimum commitment amount" is above what you were originally paying.
To rejoin the group, just click the "Accept cost increase" button on your dashboard, specify the amount of shares you want, and confirm. Our system will let your group's organizer know that you're back in.