Such as the: Terran Galactic Federation
Example of what not to do: I don't suggest naming your organization/guild/faction after a cheesy 80 show. (~cough~ Knight Rider ~cough~) I also wouldn't use the word clan anywhere in the title. This isn't feudal japan.
2) My recommendation is not to start a organization/guild/faction unless you can correlate it with some sort of story/plot line.
For Example: Shortly after human kind out grew the very planet that spawned them, they reached out for the stars. To govern this expansion and protect their now scattered numbers the Terran Galactic Federation was formed.
3) Next step is to create a logo for them.
Such as:

4) Get people to join. Yay! If you've followed these steps you organization/guild/faction probably won't suck entirely.