Well first you can add other members yourself, so they don't have to join and need any password(this is good because you don't need a password to set & you have more security that no one else can join your clan.). Therefore you need to go to your clanpage and then click on the top right on the link 'Edit Members'. Then you can easily add them by just typing in their playername & clicking save.

If you want that they can join theirself by using a password, you need to click the link 'Edit clansettings' on the top right.
Then you see all the settings you can edit on your clanpage. To set a password you need to scroll a bit down(it is before the clantext),
and type in a password. click save to save your edited settings(you can disable the password option by leaving this field blank).
Then you need to give your members the password, they can click the link to join your clan & just need to type in the password.

But like I said, it's more secure to disable password option and just edit the members yourself, because there are always guys which share the password to everyone

Thanks for asking,
JoNny