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To restrict a User API Key to specific Campaigns you need to create a Team (via Accounts | Team Management).
Add the user to the Team and set the 'Team' in the Campaign Template (Details tab).
This will give the user access to any Campaigns that belong to that Campaign Template but no others.
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Send the API Key to the user via a secure channel (google docs works well).
Add the user to the Outbound Agent' group.
Create a Team via the Team Management page and add the user to the Team (as an Agent) .
Set the 'Team' in the Campaign Template (Details tab).
A user can belong to multiple Teams and as such get access to multiple Campaign Templates.
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