Use the Bulk Actions option to tag contacts
From your dashboard, go to your contact list, click on the Bulk Actions button, and select to apply tags. You can choose an existing tag or create a new that will be applied to the selected group of contacts. You can apply tags to up to 10 contacts at a time.
Adding tags to contacts during a contact list import
While you are importing your CSV contact list, you can also add a tag to your CSV which will apply the tag to contacts. Use the Import/Upload Contacts guide to up; load your contacts with tags.
If you need to add, edit, or remove tags entirely from the contacts in bulk, you can to go your Tags settings.