Hi Vicky,
Sorry to hear your having a hard time getting into DTM.
Your workflow seems correct. Typically, when a user is invited to use DTM, they will receive an email. At that time they will be asked to log in with an Adobe ID or create a new Adobe ID. If you are then invited to join a second DTM company, you should receive the same email. Again, you can choose to create a new Adobe ID or log in with you existing Adobe ID. Usually, customers are able to log in to the existing ID without issue.
When you say logging in failed, what error message did you receive? If you want to send me a private message with your DTM company details and the email address associated with your user, I can look on the backend to see what I'm seeing in each company.
Thanks,
Jantzen