Let us say you have a lead or contact that have unsubscribed from your emails but they have requested you to add them back to your email campaigns. You can follow the instructions below to achieve the same.
Step 1: Uncheck the Email Opt Out field to true from the contact or lead details page and also remove that email from Global Unsubscribes in SendGrid.
Log into SendGrid with the account details that were provided to you.
Navigate to Suppressions -> Global Unsubscribes -> Search for the email id -> Select the checkbox and remove the selected record as shown in the following picture.