When to join a guild

Overview
There are several circumstances you have to take in when considering joining a guild, especially if you are a lower level character. There are factors such as number of members, alliances, guild halls, friendliness of members, and farming. Although these are just a few of the things to consider, those are the main points. There will also be a miscellaneous sections where some other factors will be touched upon.

Number of members
The number of members in a guild is important to your selection in many ways. If you like farming, a high-member guild would best suit you, so you have a better chance of getting people to come farm with you. If you want to be an officer, a lower-member guild gives you a better chance of being one. Some guilds will offer you an item or sometimes money to join. Be cautious, and ask about the number of members, what kind of guild (farming, GVG, etc.) it is. GvG guilds should have a higher member total than some other guilds, so if that's what you want then join one of those guilds.

Alliances
Some guilds are part of what's called an alliance, or several guilds formed into a group with one guild as the leader. This allows alliance chat, which allows you to talk to other members of an alliance without using the "all" button. A guild that's part of an alliance can take part in Alliance Battles, in which guild members fight other players that are part of an alliance. If you want that, then join a guild with an alliance. Another plus is meeting more members that are willing to help you.

Guild Halls
Guilds with guild halls are helpful because of what they have in them. There you can go to Kurzick or Luxon battles, add henchmen, and sometimes there will be merchants selling weapons, armor, (rare) crafting materials, dye, runes, and more. Also, if the guild is part of an alliance, you can visit their guild halls if yours doesn't have the proper merchant.

Friendliness of members
Joining a guild containing bad members can hurt your chances of GvG, getting help, or just having fun. Guilds with friendly members will help you with quests, missions, capping elite skills, or getting a better item. Also, talking with them while playing can be stress-relieving.

Farming
Some guilds are called faming guilds, guilds whose members come together at a certain location and kill one type of monster there for a specific item or amount of gold. Farming guilds can help earn you money as well as experience to level up. Most farming guilds contain friendly members, so aiming for one of those guilds can be a plus to you.

Miscellaneous
Be wary of scammers and cheats. DO NOT join a guild that requires you to pay them. They are usually snobs or scammers. If you feel uncomfortable in a guild, LEAVE! There are plenty of other guilds out there for you.