iPhone- How can I see who is able to pick up a shift for me?

When viewing your schedule ( after selecting am or pm), there will be a tab above the schedule for the day that says "NOT working this shift". Select this.

Here you will see all fellow coworkers who can work your shift but are not currently scheduled for the day.

If you click on the co-workers name, any possible contact information they have available will be presented. You can also send them a message through Hotschedules this way.