You can mark your foster parents by adding the 'Willing Foster' attribute to their person record. This will allow you to configure the basics of their foster profile (How do foster profiles work?) and will send them an email (if they have an email on file) to complete their foster profile through the foster portal (How does the foster portal work?)

Once you have added the 'Willing Foster' attribute, you will be able to add foster animals to their person record. (How to move an animal to and from a foster parent)