Hello Saranya,
There is an option to update the group tag of selected VSMs in the Portal.
Open the VSE tab for the VSE you are working under Monitor/ VSE.
Search for the VSMs you want to update to the same group tag. In the case below I selected all the services named with in.
Select the services and select the icon 'Upgrade the Group Tag of selected virtual services'
A new window will open asking you to provide the group tag:
After you click the OK button, you will need to refresh the VSE tab to see the updated group tags.
You can also use the VSEManager to update the group tags - VSE Manager Command - Manage Virtual Service Environments - DevTest Solutions - 10.3 - CA Technologies Documentation
The --update option lets you update the capacity, think scale, group tag, auto-restart setting, or any combination of these parameters for the named virtual service. However, you would be able to do it for one service at a time.
You can try to use the VSEManager command on a script and run it several times based on your requirements.
Hope it helps.
Heloisa