you can select the sensors by group and then just sort the resulting list after the status. Therefore got to Sensors | By Group and choose the group you want. Afterwards just click on Status in the table. This will sort your sensors and you can only choose the ones with alarm states. Now you can acknowledge the alarms.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.