To start, head over to the 'Users' section of your account.
You will see all client & staff users that have been setup with an account. Hover over the user account in question, click the ellipsis button, and choose "Edit Campaign Access."
You will then have the option to assign the user with access to 'All Campaigns' or 'Restricted.' Selecting 'All Campaigns' means that the user will have access to all the campaigns that have been created for the account, while selecting 'Restricted' will allow you to specify which campaigns you would like the user to have access to.
Once you are happy with the campaigns you have provided the user with access to, hit 'Save' to ensure any changes have been captured.