From Resources > Users you can click on External Users to see users from outside of your organisation.
Click on the Internal Users tab if you want to see users from inside your organisation.
To create a new External User click on +New
To view details of an existing user double click on them (or click on the user and then click view).
If you select a User you can make them Active / Inactive.
(It is not possible to delete a user, you would simply select them and use the Deactivate button)
If you try to deactivate a user who has tasks assigned you will see a warning:
The filter option allows you to search by Name / Profile / Customer:

To view users by a specific Project you may find it easier to look at Project > Project Home > Team, as this will show you both Internal and External users at a glance.
On the top right hand side you will see:
Export - Export a copy of the Users List in your preferred format
More Options - See below for details (Upload, Reset Password, Grant Access to Project)
Quick Search - Search by Name or click the down arrow to filter by Tags for Organizational Unit
Display - Filter by Status (Active / Inactive / All)

More Options - Upload External File
Allows multiple Users to be created by uploading a single CSV file.
Start by downloading the CSV Template:
Note: Column D is optional and can be removed to avoid confusion - This controls the section seen on the User's General Information Tab, called "Mute outgoing emails from the Platform".
Once the file has been created and saved, you can browse to select this for the File to Upload.
If you have a mixture of Profile and Calendar settings you will need to upload the CSV file in batches.
More Options - Reset Password
When the user is first created they will receive an invitation from info@hydra.cloud which will look like this:
If the user doesn't action the email soon after receiving they may get a message to say "Authentication Error Access Expired".
You can use the Reset Password option from the External Users screen to resend an email so the user has a current link.
Alternatively the user can use the forgot password option:
(Note: The Password Reset email has exactly the same wording as the Invitation)
More Options - Grant Access to Project