[wp-polyglots] What does that mean "add new user" of "an existing user on this site" ?

[Rasheed]

Hello

I am working on translating wordpress and I want to understand this point
before translating it.

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You can add new users to your site in two ways:

1. Enter the username and email address of an existing user on this site.

2. Enter the username and the email address of a person who is not already a
member of this site. Choose the username carefully, it cannot be changed. (this
one is clear)

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What does that mean "add new user" of "an existing user on this site" ?

Thanks.
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