Currently, there is no option to simply add a Mailchimp tag to an individual contact. You have to set up a report to tag individuals. It would make life easier if the indvidual can be tagged at any time. For example, if they fill out a contact form on the website, they can be easily tagged when they are added as a contact depending on the nature of their enquiry so they can then be added to relevant Mailchimp campaigns or journeys automatically.