Testing this today. A few things that appear to need improvement.
1) Adding a guest from within Planner is still not working (see post from Elijah Purcell)
2) Adding a guest can be accomplished using Outlook on the Web (BTW, WHY couldn't you guys just leave it named Outlook Web App, urrrhhh!!). This is essentially adding the user to the associated SharePoint site. The email that the guest receives does not have a link to the Planner site, all links are to the SharePoint site (see post from Alberto Ovalles). This is confusing and clunky as the proper URL has to be manually constructed and re-sent in a separate email.
It's close to 6 months since these issues were first mentioned here. Joanna, what's the ETA on getting these issues resolved?