One method is you can add tag in bulk through 'import CSV', please refer to How to import bulk contacts

Here we're going to talk about another method, to add tag directly on Audience page

Step 1:

Click「Audience」，go to Audience page

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Step 2:

Hover on one contact, then you'll find a check box, click it

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Step 3:

After choosing several contacts, click '+' at the top right

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Step 4:

Then you can choose tag from the tag list or input to create a new tag

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Congratulations!👏