You can add a tag to an account or contact that is within a List. From the Navigation Panel under Lists on the left-hand side of your screen, select List Builder. From within the List Builder window, click Run on the list that you want to work with. Check the box or boxes next to the account or contact you would like to add an activity to. Then click the Mass Actions button and select Tag Selected Items.
By selecting the Tag Selected Items option you will be presented with a new window and the ability to search for existing tags. If the tag does not exist simply hit enter on your keyboard and the new tag will be created. Tags will automatically be assigned to all items selected, and will be available on the account and contact profile pages, reporting, and for use within other lists.
Keyword Search: List Builder, How to tags, How to do mass tags, mass tagging