Feature: Enable add member button and present upgrade modal if over the limit

[nickytonline]

          @brandonroberts, just getting to review this. I think the "add members" should present the modal and not be disabled. That thought might present some extra UX decisions, so I am approving this, but would love to at minimum follow up on this in an issue. 

Originally posted by @bdougie in #3511 (review)

[github-actions]

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[brandonroberts]

Thanks. Currently, the Add Member button is disabled only because the username text field is empty.