Also please note that due to the nature of an exposed bounty, following the rules set out in the bounty broker is much more of a serious matter than in someone's own hosted bounty where things can be more flexible. Whilst the Mark can allow somethings, such as people to jump in midbattle and such, or deny people joining once combat has started - it should be kept as close to the set out method to try keep things smooth and quick.
If you're going to adjust the rules or make changes, it might be better to set this up as an own bounty to allow the mark such flexible and for warn people of such changes. It may also be better to set up the entire thing as an event if numbers push towards double digits with a much better system to handle that amount of people.
Any questions or such, feel free to contact me!