Anyone can create a guild, but it won't be visible in the guild tab until another member joins. You need to be online to accept people to your guild. There is no technical minimum number of members, but obviously a Guildmaster is needed to create the guild and send invites. There is a maximum limit of 500 members to a guild.
To communicate with other guild members, the chat buttons are
# is the order number of that guild (which can be found in the guild tab).
Guilds are permanently affiliated with one Alliance, which is selected when creating the guild. When fighting in Cyrodiil you can claim resources for a guild, though your character's and the guild's Alliance affiliation must align in order to do so. If you capture a fort you can claim it for a guild too, which unlocks some guild bonuses.
There are four ranks in any guild: Guildmaster, Officer, Member, and Recruit. The ranks can be renamed, and up to six more can be created. By default the lower ranks have no permissions (except for Officers being able to send invites), but the Guildmaster can assign permissions to each rank. These permissions include resource management and access to the guild bank and store. Only the Guildmaster can promote or demote player ranks, and assign permissions to each rank. There can only be one Guildmaster, but someone can be promoted to that position at the cost of the current Guildmaster being demoted.
Disbanding a guild is possible. To do so the Guildmaster must leave the guild before all other members, whereupon the guild will vanish when the last person present leaves. Any items left in the bank will be destroyed. If the Guildmaster becomes inactive and the other Guild members wish to keep the Guild alive, they must contact Customer Support.
To open a bank for the guild it must have a minimum of ten members. In the guild bank, money and items can be shared with members that have access. 500 items can be stored and shared with the guild members.
For a guild's store to be unlocked and usable, it must have a minimum of 50 members. Players set their own prices when putting items up for sale in the store. You can have up to 30 items listed in each of your guilds' stores at a time. Every item you put up for sale will remain in the store for a maximum of 30 days. If no one buys it during that time, the listing will expire and the item will be mailed back to you. Additionally, you are also able to manually cancel listings, which are also sent back to you by mail.
There are several fees associated with selling items through guild stores. First, you will be charged 1% of the price you want to sell the item for upon posting it. This collateral fee is refunded to you if the item is sold. Second, when an item is sold, 3.5% of its posting amount is automatically deposited into the guild's bank as a tax for using the store. Last, another 3.5% is cast directly into Oblivion, never to be seen again. This means that you always get 93% of the gold you ask for when items sell, and lose 1% when they don't or you cancel them.
Only guild members are allowed to put stuff up for sale in the guild store, and by default, only members can browse it. This is done by selecting the Guild Store dialogue option at any Banker. However, people who aren't part of a guild may also able to access the store if the guild has hired a Guild Trader through bidding or if the guild controls a Keep in Cyrodiil. It is possible for a guild to have their store displayed through both of these simultaneously. Members of any alliance can access a guild's store at their hired Guild Trader, but only members of the same alliance as the guild that controls a Keep can access the store at the keep's Quartermaster.
To have a guild trader display your guild store to the public, you must first place a bid at your desired trader location. Guild Traders are scattered all across Tamriel. Once you have made a bid, gold is deducted from the guild bank and your guild becomes attached to that specific trader until the guilds are switched. For the rest of the week, you will only be allowed to update your bid at that location. You are free to raise your bid to a higher amount at any time, but it must be for the same guild trader. You cannot change your mind and remove a bid or lower its amount once it has been placed. All bids are blind, meaning that there is no way to know how many other guilds have bid for the same guild trader as you, or how much gold they have bid.
The guild traders are switched on Tuesdays at 3:00pm EDT for North America, and at 20:00 GMT for Europe (with all bidding closing 5 minutes prior.) After this point, traders begin displaying the stores of whichever guilds bid the highest for each of them. The new stores will then be accessible by the general public for an entire week. If your guild was outbid, your bid will be refunded back to the guild bank. After the trader switch, if not even a single bid was made for a certain guild trader, anyone will be able to walk up to that trader and immediately hire them for a set amount of 10,000 gold. This process is first come first served.
The Heraldry system, unlocked when your guild has ten or more members, allows the guildmaster to design tabards for their guild. Tabards are widely customizable, with numerous background shapes and patterns, and a vast array of crests, all available in various color shades. Once created, the Guild Tabard can be purchased from the guild's store, even if the guild doesn't have 50 members. It is always found alone on the first page when initially accessing the guild store.