Can someone who is not on Loop Email join a Team I create?

Yes, you can invite someone who is not using Loop Email to join a Team you have created on the platform. 

Just like other members of the team, people who are not using Loop Email will be able to send receive emails and messages which are being discussed in the team.

Sending and receiving messages is all done via email. Therefore someone who is not on Loop Email will only receive your Team Message as a traditional email - The email message/email is sent via an automatically generated email address to make it easier to identify that it's from a Team and not one single person.