There are two ways to reach out to candidates you find on SeekOut: Messaging Campaigns, and ad-hoc emails.
I want to engage with multiple candidates at a time.
SeekOut helps you identify candidates, find their contact information, and engage with multiple prospects at once with messaging campaigns. See the article SeekOut Messaging Campaigns for steps on how to create a messaging campaign.
I want to contact a single candidate.
You can also send ad-hoc messages to individuals based on a messaging template you create. See How do I send a single email to a candidate? for information on how to do this.