You can send influencers messages once you have added them to any of your campaigns. Click the Messages option on the left sidebar to contact influencers via the messaging inbox.
Alternatively, you can click the Campaigns option on the left sidebar then click on the name of the campaign that features the influencer you want to message. From the individual campaign screen, find the influencer in the list and click the mail icon to send a message.
Note: You are limited to 3 messages before a pitch is accepted. Once a pitch is accepted, this limit goes away.