If an Outlook user is using a shared inbox to communicate with their members, they will find that simply authenticating with their personal account will only add their personal inbox. It will not add any shared inboxes that they are a member of.
In order to enable a user of your tenant to add a shared inbox, you will need to allow the user to log into the shared inbox directly. This is a simple 3-step process as outlined below.
Directions
Find the shared inbox in your Users list.
2. Click into the shared inbox to view its properties. You will need to ensure that under settings, "Block Sign In" is set to "No".
3. Finally, simply reset the password to this inbox for the user who you want to manage this inbox. At that point, the user can go back to Aspire and connect the shared inbox by following the steps in this article.