Use a bulk action to tag multiple subscribers at once.
To tag your subscribers, go to Subscribers under the Grow tab in the navigation.
Scroll down past the Subscribers graph to view your list of subscribers. If you want to tag subscribers who have a certain Tag or are in a certain Segment, click this Tag or Segment at the right side of the Subscribers page to view just these subscribers first.
Next, you'll see a checkbox to the left of each subscriber. Click these checkboxes to select (or deselect) the subscribers you want to tag:
Alternatively, mass-select all subscribers on the page by clicking the checkbox on the top of the subscriber list:
To select all subscribers in the list (if it spans multiple pages), click the Select all # subscribers option that appears after you select all subscribers on the page.
Once you've selected your subscribers, click Bulk Actions, followed by Add Tag from the dropdown menu.
From the window that pops up, select the Tag(s) to add to your subscribers:
Hit Save, and that's it. Your subscribers are now tagged.
NOTE: If you're tagging a large number of subscribers, the action may take a while to complete. Kit usually processes bulk actions at a rate of several hundred subscribers a minute.