adamrice / #calendar

https://gath.io/

Lightweight site for creating events, sort of like evite

http://foulab.org/en/user/strawdog/Calendar

Includes script for generating your own.

https://mykolab.com/

fee-based, supposedly secure and private mail & calendar service based in Switzerland

http://www.doodle.ch/main.html

Seems like the lightest-weight group scheduling gadget