I've added two new commands:

CLAN SET MEMBERSHIP OPEN/INVITATION (clan head only)
- Toggles whether the clan is open to public membership or only by
invitation. Open membership allows anyone who is not an enemy to
type CLAN JOIN <clan> to join the clan.

CLAN JOIN <clan name or alias>
- Join a clan that is open membership.

You can find open membership clans through HELP CLANLIST. I can imagine some fun roleplay opportunities being possible with this.

Enjoy!

Penned by my hand on Closday, the 8th of Haernos, in the year 490 MA.