You can add users from company directory by clicking the button next to email address text field and select the associated company domain(s) from the list.
For adding members to multiple projects request, I would recommended you try export team member to excel from one of the project that you would like to use as template under Team tab. After updating the Excel spreadsheet and add users in the list, then use "Import from Excel" feature to import multiple users at once to a project one-by-one.
We will pass your questions to our product team to review and improve the project team invitation process going forward and please feel free to let us know if you have any feedback.