Multiple team members here have experienced an issue where when they open the accept invitation link and fill in the form's fields the "Accept Invitation" button is not enabled.
This is happening on Windows 7 across Chrome latest, Firefox latest, and IE 11.
Workaround seems to be to not only just tab off the captcha field, but you must tab off of all fields starting with username. By "Tab Off" I mean select the field and then press the tab key repeatedly. My best guess is the validation isn't triggered if you simply click out of the field.