You can add a contact to a campaign via three methods (CSV upload, Zapier, public form). Each campaign has a unique public form that does not require authentication which can be used to add contacts to the campaign for sending:

On the 'Campaigns' tab, you'll see a 'Contact Form' link on each campaign row. Click this link to view the public form for this campaign:

This form can be used by internal team members, or really anyone, to add contacts into the campaign.