Currently, it is not possible to create account users from an account's "Users" tab; it's only possible to assign existing account users. Account users must be created from the "Account Users" portlet. This was done for expediency.
However, Liferay Portal has a pattern for creating a new object when selecting from a list of existing objects; namely when assigning a segment to a regular role.
Using this pattern, the modal for selecting existing account users will include a blue "plus" button for creating a new account user.
Starting from an account detail's "Users" tab and clicking on the blue plus "Assign Users" button:
- The "Assign Users" modal includes a blue plus button for creating a new user.
- Clicking on the plus button navigates to the account user creation form. (Note, no need to select an account like how the Account Users portlet works)
- Saving the user creation form navigates back to the modal and the created user is automatically selected. (Note, the selection checkbox will be disabled since the user will already be assigned to the account at this point. This is the same behavior for how existing account users appear on the table).