A party lasts as long as its members stay together. A guild is permanent: a name, a crest and a chat channel for a group that keeps playing together.
Founding it
Guilds are founded at the guildmaster in town. Right click him and open the Guild Hall. It lists what founding asks of you, with a tick beside everything you already have. Founding asks for a seasoned character and some gold, and you cannot already be in one.
Give the guild a short name and pick a crest to go with it. The name is yours for as long as the guild stands, and it is what everybody will be typing, so take a minute over it.
Bringing people in
Invite somebody by name from the guild panel, or type /ginvite and their name
into chat. They see the invitation and decide. Nobody is added without saying
yes.
Ranks
The leader can do everything: invite, remove, set ranks, edit the guild and disband it. Officers help run it, inviting and removing members and keeping the guild's details up to date. Members turn up and are part of it.
Two rules are worth knowing before you hand out ranks. You cannot act on somebody at or above your own rank, so officers cannot remove each other. And promoting somebody to leader hands them the guild, making you an officer in the same moment, because a guild always has exactly one leader.
What it gets you
A chat channel of your own. Every member can talk to every other member wherever they are, on any map and in any party. Your guild also shows on your profile, so anybody who looks can see who you run with.
If it does not work out
The leader can disband it. The name becomes free again, and what it cost to found does not come back.