On the Extended server (Freeport) Silver+ members can buy a guild charter for $10 (1000 SC), then you need 6 more people of the same alignment in a group to start the guild.
On a Legacy server (every server other than Freeport) you need 6 people of the same alignment in a group and to speak to a guild NPC to start the guild (no extra step of spending RL money for this basically).
Edited, Oct 8th 2011 10:07am by Cyliena