If I sign up for the iOS Individual Developer Program (rather than the Company one), will I be able to set up multiple users within iTunes Connect so that I can have some TestFlight internal beta testers?
Sure. This is an extract of iTunes Connect Developer Guide:
Enable internal testers in Users and Roles.
To be eligible to become an internal tester, a user must be part of your iTunes Connect team with the Admin, Legal, or Technical role. Adding iTunes Connect users and setting their roles is explained in Setting Up an iTunes Connect User. You can see users’ roles in the Users and Roles section.
Enabling an iTunes Connect user for TestFlight beta testing is explained in To enable a user as an internal tester.
Add internal testers to your app.
The Internal Testers pane in the Prerelease section lists the current users who are enabled as internal testers on your iTunes Connect account.
Select up to 25 of them to add as internal testers for the app and click Save.
To invite internal testers, select the Builds tab and set the TestFlight Beta Testing switch to the On (green) position. The users you selected will automatically receive an email inviting them to test. They will be directed to use the TestFlight app on their iOS devices to accept the invitation to test the latest build available.