Test resources are managed at the team level rather than on the account or organization level, so you'll want to create a team in order for team members to be able to use Flood.
By default, your name will show up in the “Assign Team Manager” field. You can click on the X icon to remove your name, and input email addresses of other people who you want to invite to become Team Manager of the new Team. Tips: you can leave the “Assign Team Manager” field blank at the moment of creating team. This means that the newly created Team will have no member. You can then invite members later.
The side sheet will close and you will see the new Team. The invited Team Managers (if any) will receive an email saying they have been invited to become the Team Managers.