MAKE IT BIG!
To get people's attention at church, I use large pieces of roll paper, 36" wide, and I cover 2 banquet tables with the paper ( one for each half of the year, note including summer.)
Across the top, I write columns with the workshops, then for rows, I write the story and all the dates for each rotation. This makes a large grid, with boxes for the person to write in their name for a workshop!
I write in the activity if I know it, but as other people have said, folks usually base it on their schedule.
Other suggestions:
Leave a space on the calendar to place special cookies or coffee cake which a member gets for signing up during coffee hour. Bribery works, and it gets the point across. 
