Click Connectivity -> Extensions
Click on "Quick Create Extension" or "Add Extension".
Quick is fine for more users, unless you have more specific requirements.
If you use Quick Create, you can just edit the extension and click on their "secret" field to find the password.
To make your SIP softphone or real register use the following info:
SIP Server: Your IP/Domain of FreePBX
Username: Extension# (eg 1234)
Password: the secret
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