What Does the Order Do?
The Order of the Phoenix is a society aimed at protecting those who cannot protect themselves, and have sworn to prevent any 'evil' from harming the Wizarding World. They counter the actions of the Death Eaters, be that through dueling or merely stopping the move they attempt to make. They hold meetings, at which the current leader and their council (advisors of sorts) meet with all of the members who can attend that day. They discuss plans, make suggestions, and wait for a decision to be made before advancing on any ideas.
What is the Order?
The Order of the Phoenix is a ‘Good’ Organisation, which is aimed at fixing the wrongs in the world. They serve as vigilantes, a voice for the under-represented or oppressed, and act as the opposite to the Death Eaters.
History of the Order:
The order of the Phoenix was founded by Albus Dumbledore during the first wizarding war, as a means to fight against the Dark Lord and his followers. Following the downfall of Lord Voldemort, the order dissipated, and the many witches and wizards of the ranks went back to their day jobs, with many becoming Aurors to continue fighting the darkness. Shortly before the begining of the second Wizarding war, the order was restored, and it worked hard to prevent the return of Lord Voldemort to his full power. During the first battle of Hogwarts, the order came to its assistance, providing most ot the defence it received. The Order had hoped to be able to relax after the battle ended, but the Death Eaters rose to importance again, and the fight goes on.
Duties as a member:
As a member of the Order, you ought to attend meetings and participate in battles - particularly in mass fight threads. Both the meetings and the battles provide chances for you to get involved and make a difference.
How to Join IC:
Your character needs to know someone within the organization to join, otherwise they will not have the ability to enter Grimmauld Place. This can be the leader or any of the other members. They don't necessarily have to write out their joining, but ought to know who brought them in. Referencing that in a post is as good an entrance as any.
Rules of Grimmauld Place:
There aren't many specific rules here, other than the fact that, if you want your character to have a position on the council, you'll need to check the Positions list, run by the Admins. It's a list of roles you can pick from or petition to add to, where the duties of those members are described.
Other than that, if you post in the house, I suggest that you mention what room you are in during your introductory post, so others know where things are happening and can orientate themselves within the building. Typically, only Order business will be discussed within Grimmauld Place, but that may change as the plot progresses.
Thanks to Jess Potter, who created the template and provided part of the information above.