Hi Erika:
I have invited Dennis to join another project and he should now be able to go to https://bim360plan.autodesk.com and sign in. If he doesn't have an Autodesk ID, he will need to register for one with his work email address. He may need to use the project selector to access your project.
The problem may be that his invitation to your project has expired. We used to expire these invitations after 3 days - we increased that to 60 days back in February.
mike.d
Head of BIM 360 Product Management & UX