While I agree that would be a nice feature to have, it’s simply not possible with the current database structure of this plugin (which was born as a fork from another plugin that had already setup the basic database table structure).
It would require some fairly massive re-writing of the code, or, at the very least, creating a new sheet type with its own separate database table and separate functions to handle all the display and sign-up functions (both on public and admin side). Either way, still quite a bit of extra coding to make it all work.
Something specific, like what you need with 4 volunteers for every evening for every day of the year, is probably better off being handled by a custom programmed solution, like I have done for other organizations.
I will leave this here, along with all the other feature requests, although the chances of me adding something like this to this free plugin (which generates no revenue or even donations), are extremely small. But, since the plugin is open source, other programmers may want to take some of these feature requests ideas and run with them.