I'm having trouble inviting members to boards who have Outlook as their email host.

[Lee Iben]

This is problematic for two reasons:

1. I've already paid for their access and they are not getting the confirmation link.

2. I'd like to recommend Trello to clients to use but my confidence is low because of this issue.

[Trudy Claspill]

Hello @Lee Iben 

Welcome to the Atlassian community.

Do the user who are not receiving the confirmation link have the same email domain?

Take a look at the Answer on this post for guidance on dealing with users not receiving the invitation email.

https://community.atlassian.com/t5/Jira-Software-questions/An-User-does-not-receive-the-invitation-mail/qaq-p/1923892