Slack requires an enterprise plan to invite individuals via API but you can still use Headcount to send invites for Slack using the manual Slack invite option.
​
Simply navigate to your Slack members page under Administration -> Manage Members.
​
Then select Invite People.
​

Once you've copied the invite link copy it into your Headcount account.
​
​

Once you've added your link, click Update and then you can invite contractors to apps you use.
​
Make sure you only add the invite link and not any of the extra text added.
​