When you increase the price of your group, everyone who already joined the Pay By Group will have to re-approve their share of the new cost.
NOTE: This is not the case if your group is splitting the price flexibly, where everyone specifies their own share. Under this split type for the group, you simply need to get more people to join the group to commit the extra money.
When you increase the price, if people will need to recommit you'll see a warning like this:
If you do choose to increase your price, you can see the number of people who still need to accept the increase on your dashboard.
Lowering the price will not require anyone to re-commit or accept any additional charges.