User Can't Be Added to Group

I have added a user to a PowerBI group, and they have signed up to PowerBI through the email invite. Upon logging in, they do not have access to the PowerBI group.

If I look in the group administration panel, I can see that the user's status is still showing that the email invite has been sent. If I attempt to remove the user, using the 'x' next to their name, and press save, the user returns once I open the group administration panel again - in the same invite-only state.

If I remove the user, and re-add them, PowerBI recognizes them and their account, and the user appears to be added successfully. When I press save, the user still returns once I open the group administration panel again - once again in the invite-only state.

In summary: I have invited a user, they have signed up, and yet they cannot access the PowerBI group to which I have added them. I cannot get them to be properly added to the group.

If I remove and re-add the user without saving in between, I get the following error:

Something went wrong.
Couldn't update the group
Activity IDbf732bc3-690e-4562-89b5-2f88f310d424
Request ID33d3f72f-9e19-7556-f6ea-6749942c1366
Status code400
TimeWed Feb 03 2016 10:36:46 GMT-0800 (Pacific Standard Time)
Version13.0.800.375
Cluster URIhttps://wabi-west-us-redirect.analysis.windows.net