To bulk add contacts to an Automation, search the contacts you want to add to the automation within CRM → View Contacts. Then, click the Actions button, and select Add to Automation.
Then, select the name of the Automation (these are configured in Marketing → Automations) and choose whether you want to start the automation now or later. If you choose Start A Later Date, a calendar widget will open, allowing you to select a date. Then select Okay.