Qi Jang,
I know we had a conversation about this on another post, but wanted to check here if that answer helped. I'll add the basic information here if others may need it.
There is a Knowledge Base Article, How to create a user with Web Services API, that has an example using curl and some further information.
Basically the payload is:
{"User":{"UserName":"emailformat@ wsapi.com","EmailAddress":"validemail@ domain.com"}}
Posted to the endpoint:
https://rally1.rallydev.com/slm/webservice/v2.0/user/create
Thanks.
Michael