Only the account Owner and Admin users are able to invite other people to the team. They can do this from within Team Settings after clicking your avatar at the top-right of the interface.
Open Team Settings
Click Users then enter an email address and click Send Invitation.
Note: As soon as you send an invite the user will appear as a resource inside Workstack. When the user logs in for the first time they'll be able to finish their profile.
Creating Test Users
Although Workstack itself does not allow you to create Test Users you could, however, use a service like www.mailinator.com that allows you to setup temporary test email addresses.
This way you can send an invite to the test email address from your Workstack account and accept the invite there. If you are testing this way we recommend that you use a separate browser for each logged in test user as you won't be able to login with more than one user account using the same browser.
Once you are finished testing you can then delete these users from your Workstack account within Team Settings.