There are a few ways to add contacts to a campaign:
1. one at a time - you can either add a contact one at a time on the campaign page by adding contact to campaign or on the contact record page by adding to campaign
2. Multiple at a time - on every page that lists contacts, you will see checkboxes on far right and a campaign button on the top. You can select the checkboxes for the contacts you want to add and then click the ADD TO CAMPAIGN button and select the campaign you want to add the contacts to.
You can also do this on the search results page if you want to run a particular search and add the search results to a campaign.
3. Add during import process - the easiest way to add a large number of contacts to a campaign is probably during the import process where on the field mapping page you can select to add the imported contacts to a campaign.
If you have already imported the contacts, you could actually import the file again just for adding to the campaign because we will not create duplicate contacts during the second import and only assign to the campaign because there can only be one contact created for each unique email address.