Clan Wars are month long battles with a team of 10 players where you meet specific requirements to earn rewards. As of October 2019, clans no longer have a minimum requirement of 10 members. A team may be comprised of 1-10 players. But all players must reach the goal for prizes to be awarded.
The basic information on clans is on the Clan Page at Munzee.
Each month the requirements are different. Sometimes they will require you to cap certain types of munzees, some months will ask you to deploy a certain amount, sometimes it's based on your activity, how much you cap and deploy. Example
Clan requirements come out in the blog (and usually posted in several Munzee FB groups) at 3:00pm MHQ time on the 28th of each month. When they come out, look over the requirements carefully and decide which level you will be able to accomplish that month. Most months have both an individual requirement and a clan requirement. You MUST complete the individual requirement. You should also do at least 1/10 of the clan totals.
Once you have determined what you are able to accomplish, you can go to the Clan Transfer Market or Munzee Clan War Information Group and request a spot in a clan that is going for that level. There is also a channel in the Munzee Discord to look for clans. You will be expected to complete at least the minimum requirements for that level during the month. Clans can continue on to the next levels if all the players agree. You can now also search for clans in the app, using the clan management button on the map. (Orange crossed swords above the blast indicator on the right.) Look for clans marked with the level that you wish to achieve. Remember, that clans still have the option of reaching for higher levels, regardless of how they are marked.
I personally do not recommend looking for clans through the app. It's best to communicate with the clan leader before joining a clan so that you know that you are on the same page. Often people randomly join a clan only to find they have joined an inactive clan and their month is wasted. Communication is absolutely key to the success of a clan. Do not join a clan or accept a new member without communicating your planned level for the month. It is recommended that your clan be set to "private" or "hidden" unless you are willing to take a chance on members joining your clan and not meeting goal. With "private" you have a chance to communicate with the prospective new member before accepting them. Once a member joins a clan, and clan war has started, they can not leave until the end of the battle.
Clans will only reach a level when all members have reached their requirements for the level. Once the requirements for a level have been reached, you will receive a notification to collect your rewards in the app.
Some clans stay together each month and vote on the level they will go for as requirements come out.
There are also clan groups such as World Wide Munzee Hunters, Jibby, etc. These groups have FB pages and several levels of clans. As requirements come out, players in the group put in a bid for their desired level on the FB group for the clan group. The organizer of the clans groups will then fill the clans based on those requests. Some compromise may be expected as they try to fit everyone into the clans.
All invitations and requests are handled in the app, as of October 2019.
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