That's an option in employee classes. In the configurator go to the Employees tab, Employee Classes button, Clock In / Sign In tab. Select your bartender class and uncheck the "Sign out after transaction" option.
You'll probably want to enable the Auto Sign-Out for your workstations. This allows the workstation to sign people out if its been idle for a set amount of time. In the configurator go to the Devices tab, User Workstations button. Do the following for each of your bar workstations:
On the General tab enter an Auto sign-out Delay. This is the number of seconds the workstation will remain idle before logging someone out.
On the Options tab check the Enable Employee Auto sign-out option.