The CO-WORKERS Tab is where (if your company allows) you go to look at your co-worker's schedules. It only enables you to look at Co-Workers who can work in your Work Group. So, even though you may have a Co-Worker that has the required skills to work a shift for you, they may not have the permissions in RosterApps to work in your Work Group, and therefore can't pick up your shift.
You can select which of the Workers who can work in your Work Group in the select box on the left-hand side of the page.
You can also navigate to whichever week you would like, or to step forward or back in the weekly display.
Some companies do not allow workers to view their Co-Workers' schedules, largely because of privacy concerns. If this is the case, you will run into an error.