When you import a spreadsheet, you're given the option to choose a tag to associate with that group of people. While you can import your entire community in one spreadsheet, you might find it more helpful to break up your list into separate spreadsheets for each group, which can then get separate tags.
You can also only import a tag, for people already in your community. Simply create a spreadsheet with the emails of those people you would like to tag (for example, people who were invited to last year's gala). If you have some people in the sheet who are not yet in your database, you will want to add additional columns for important information like email, phone, and address. If everyone in the sheet is already in your community, you only need the Email column. When you import your new sheet, select the tags you would like to associate with that group (for example, 'Invited to Gala 2014'). Because those emails are already in your community, nothing will change, except those individuals will receive their new tag. Repeat with new tags and new lists of emails as often as needed!