When to join a guild

Overview
There are several points you have to keep in mind when considering to join a guild, especially if you are a lower level character. There are factors such as number of members, activity, alliances, guild halls and Capes, kindness 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 section 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, High-end PvP, High-end PvE, etc.) it is. If you are looking for a guild with a lot of members, your best chances lie with an active PvE guild.

Alliances
Some guilds are part of what's called an alliance (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. A guild that is 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. An alliance can have two alignments: Kurzick or Luxon. Make sure that you join the alliance of the alignment you want. Also keep in mind that some guilds want you to donate a specific amount of Luxon/Kurzick faction each day/week/month.

Guild Capes
When joining a guild, your character will get to wear a cape with that guild's emblem, if the leader has bought a cape. You may choose to show or hide your cape. The cape design can only be changed by the leader.

Guild Halls
A guild hall is a boon to any guild, as it provides numerous easy-to-access NPCs and PvP opportunities. With a guild hall, you can participate in Kurzick or Luxon battles, form a group for Guild-versus-guild battles, or visit various merchants selling weapons, crafting materials, dyes, runes and more. These merchant services may be bought at the Guild Lord. Also, if the guild is part of an alliance, you may visit other allied Guild Halls if yours doesn't have the merchants you require.

Kindness of members
Joining a guild with "bad members" can hurt your chances of GvG, getting help, or just having fun. Guilds with friendly members will (relatively) help you more with quests, missions, capturing elite skills, or getting better items. Also, talking with such members while playing can be stress-relieving.

Farming
Some guilds are farming guilds: guilds whose members come together at a certain location and kill monsters for a specific item or amount of gold. Farming guilds can help you in earning money and getting experience to level up.

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.
 * Guilds which are recruiting use several terms in their speech: TS (TeamSpeak, VoIP program), Vent (Ventrilo, VoIP program), Site (which just means they have a website for their guild), GvG/HA/Codex (this means they are a guild usually busy doing Guild Battles, Heroes Ascent battles and Codex Arena battles), PvE active (a guild that has a lot of PvE-ers), PvPvE or PvX(a guild that is doing active PvE as well as PvP), GH (the guild has a Guild Hall), and FFF/HFFF (The guild participates in Faction Farming, for those with the Factions campaign).
 * Guilds may specialize in certain areas such as FoW (The Fissure of Woe), UW (Underworld), or DoA (Domain of Anguish).