Today the best way of communicating with people if what you want instant communication is SMS. It tends to be read immediately. Often I find it takes up less time than a phone call. I saw a survey that showed that SMS tended to be read 98% of the time and almost all within seconds of being sent, in comparison emails gets about 22% reads and often not for days.
This in retail makes it invaluable for rostering.
Here are two tips you can use.
1) People particularly if they are casuals tend to forget shift, you can use SMSs to remind them of shifts a short time before they are expected.
2) One of my clients, when the shop gets busy and, needs someone immediately, what he does, is sent out a few SMSs to his causals. The first one that responds gets the job. This he finds an efficient way of finding someone and takes only a few minutes to do.