If a user provisions a Dynamics 365 Preview instance (and presumably soon Trial instance) and then another user attempts to join that organisation their join request sometimes fails.  It's not clear what causes this - (perhaps if the user tries to join before the Dynamics 365 Setup Wizard has been successfully run?)  In another scenario the join request is successful but the email to the provisioning user is (for what ever reason) not received.

In either of these circumstances the joining user is unable to re-submit their join request (see attached screenshot) and the provisioning user is unable approve the original join request.  This appears to be a problem with all Trial and Preview instances.

In addition another problem with at least the Dynamics 365 for Marketing Preview means that it is impossible for the provisioning user to invite new users from within the Marketing App.