I would do this by having a field where you type the text message using a formula that includes the IF statements. I haven’t used the SendGrid block though, Im using Twilio and Zapier to do the same.
There are SO many ways to do this :). If you already have a view with volunteers names, email address, and checkbox columns for Mon, Tues, Wed, Thurs, Friday. The easiest thing I think would be to create a formula field that returns the days of the week that each person works, for instance a field named Working that has the following formula structure:
Concatenate(If({mon},“You are working Monday”,“You are NOT working Monday”), if{{tues},“You are working Tuesday”, “You are not working Tuesday”),…)