All that is required to join a guild is for a character to have reached 40 hit points and for someone who is a ranking member in a guild to give you an "invitation".

When offered, this invitation will automatically appear in your inventory as a scroll. You simply "use" the scroll to join.

You will receive confirmation in your text window that you have received an invitation and confirmation if you use that invitation to join. You may choose to let the invitation expire automatically 5 minutes after it is offered if you do not wish to join a guild.