It's not possible for employees to add their own shifts to the schedule, but you can allow your employees to choose their own schedule by posting available shifts.
Employees matching the location and position of an available shift receive a notification that it's up for grabs and can log in to apply to work it.
If you want to allow employees to pick up shifts at-will, you have the option of allowing those shift applications to be automatically approved. This means the first employee to apply for the shift will get it.
Go to Dashboard > Settings > Company Settings, then scroll down to Available shifts and shift offers. Switch the toggle to "Yes" for Automatically approve applicants, and as soon as an employee applies for a shift, it will be theirs.
If you would like to review applicants for each shift so you can choose the best person for the assignment, simply leave the setting on "No" and you will receive notifications for all shift applications so you can choose which employee gets each shift.
Want to learn how to approve more than one employee for the same shift? Click here.