The method for subscribing a contact depends on when you need to add them.
After the campaign has already been launched and you want to add a contact:
Click on the Contact Icon on the left toolbar and select "All Contacts."
Find the contact that you want to add to your existing campaign.
On the contact page, find the plus "+" Sign next to the campaigns block towards the bottom.
This will show you a list of the ongoing and future campaigns. Each of them has a subscribe button with a "plus sign" that you can press to add the contact. It will turn green and say "Subscribed."
At the moment of launching the campaign:
After setting up your new campaign, you will have the option to add subscribers to it.
At the bottom of the setup page, there is a button "Assign Subscribers."
You will see a list of your contacts.
Find the one you want to add and check the checkbox to the left of it.
More than one can be added simultaneously.
Hit the actions dropdown above the list and select "Subscribe."
If the contact does not exist yet, you can click "Add Contacts" using the button to the right-hand top and select "Add Individual Contact" from the dropdown.
For a full tutorial on launching a campaign, please read this article.