You need permission to create users. The Admin role has it.
Go to Settings > Users and select New user.
In Create user, enter their Name and Email, and pick a Role.
Save. UDP emails them a temporary password.
[Image: The "Create user" dialog with Name, Email and Role]
They sign in with it and choose their own password. If they need a different set of properties from the rest of their role, edit the user after creating them.
Messages you might see
A user with this email already exists: they already have a user. Find them in the list and enable them if they're disabled.
Cannot assign a role that grants permissions you do not have: you can only give roles whose permissions you have yourself.