You can easily add tags to clients from either the add client flow, the client list view, or the client's profile view. Once you've tagged a client with a certain tag, you'll be able to sort your client list view by that tag.
In the "Add New Client" form found by going to the Clients menu and clicking the green "Add New Client" button, you'll see the field for customer tags underneath your account's custom fields:
In the client list view, if you select multiple customers you'll see a button at the top of the list to "Add Tags". If you click on it, you'll be able to add tags to the customers in bulk:
Lastly, on a client's profile, you'll see the tags section in the "Client Details for Staff Use Only" section:
You can use any of these three options to add tags to your clients. They all work the same way, it just depends on what's most convenient to you at the time.
When adding tags, if you start typing in a tag name and that tag already exists on your account, it will appear as a suggestion underneath the type/input field:
You can use your up and down arrows on your keyboard to select the tag from the list of suggestions shown there or you can select it with your mouse. Once you've selected the tag to add to the client, it will appear in a small blue tag box underneath the input field:
If you start typing in an entry for which there is not currently a tag on your account, you'll also see the option at the bottom of the suggestions list to add a new tag:
The same tag can't be added multiple times to a customer's profile. For example, you'll notice that the client in the image below already has the tag "Preferred" applied to her:
If I try to edit that client and add "Preferred" again, I'll get a warning that there are no new tag matches:
You can add as many tags as you'd like to a client. Once you've entered all the tags you want to apply, just click the save or update button depending on which method you're using
Once you've updated the customer's tags, you'll see them appear in the customer tags field for whichever customer you applied them to:
If you added any new tags while you were tagging the customer, you'll notice that those new tags will now also appear in the display tag dropdown at the top of the customer list: