I like the following ideas as well:
Signing up and not showing up - Penalty of 0.5 points
Signing up at least 2 days in advance, and showing up - Reward of 0.25 points
Leaving early without warning - Penalty of 0.5 points
The effects of these are small individually, but over time they strongly encourage people to get in the habit of a) signing up, and b) sticking with their plans. Furthermore, it doesn't penalize someone for not signing up at all.
Signing up and not showing up - Penalty of 0.5 points
Signing up at least 2 days in advance, and showing up - Reward of 0.25 points
Leaving early without warning - Penalty of 0.5 points
The effects of these are small individually, but over time they strongly encourage people to get in the habit of a) signing up, and b) sticking with their plans. Furthermore, it doesn't penalize someone for not signing up at all.