There are multiple ways to put someone onto a campaign depending on how you want to launch your Campaign.
If you are starting communication with a prospect or customer, you can use Add to Campaign​ from any page in ToutApp.com or from Salesforce.
This flow will allow you to preview, edit and schedule your first email in the Campaign and if your Campaign starts with a task - these tasks will get queued for you in your Tasks list in ToutApp (and Salesforce).
You can do this from the Campaigns tab in ToutApp.com:
You can find the Add to Campaign button on the Conversations and People pages in ToutApp.com:
If, however, you are following up with a prospect or customer from your Gmail or Outlook inbox, you can add them to a Campaign via your compose window when you follow up with them if the first step in the Campaign is an Email..
Just 'Reply' to their email and choose a Campaign from your Tout plugin: