Create users using webapi

Start with official doc... it's pretty simple and you have on hands  tutorials about it...

docs.appian.com/.../Web_API_Tutorial.html

Please follow the below link. 
docs.appian.com/.../Ways_to_Start_a_Process_From_a_Process.html

When creating a new web API, there is a template to start a process.

Thanks for the guidance...I created webapi using start process template and it is working fine..Next concern is how the parameters(wwid,firstname,lastname,email,role)which i passed in body  will come from external system to webapi?also i need guidance how it can be authenticated

a!localVariables(
  local!data:if(
    isnull(http!request.body),
    {},
    a!fromJson(http!request.body)
  ),
  a!startProcess(
  processModel: cons!MDA_ADD_USER,
  /* A dictionary or map of parameters passed to the process model */
  processParameters: {
    wwid: local!data.wwid,
    firstname: local!data.firstname,
    lastname: local!data.lastname,
    email: local!data.email,
    role: local!data.role
  },
  isSynchronous: false(),
  /* This expression will run when the process is started (when run asynchronously) or completed (when run synchronously) */
  onSuccess: a!httpResponse(
    statusCode: 200,
    headers: {
      a!httpHeader(
        name: "Content-Type",
        value: "application/json"
      )
    },
    body: if(
      a!isNotNullOrEmpty(fv!processInfo),
      a!toJson(fv!processInfo),
      null()
    )
  ),
  /* This expression will run if the process fails to start */
  onError: a!httpResponse(
    statusCode: 500,
    headers: {
      a!httpHeader(
        name: "Content-Type",
        value: "application/json"
      )
    },
    body: a!toJson(
      {
        error: "There was an error starting the process"
      }
    )
  ),
  /* If the process is started synchronously, this expression will run if the process does not complete in 30 seconds */
  /*onIncomplete: a!httpResponse(*/
  /*statusCode: 504,*/
  /*headers: {*/
  /*a!httpHeader(*/
  /*name: "Content-Type",*/
  /*value: "application/json"*/
  /*)*/
  /*},*/
  /*body: a!toJson(fv!processInfo)*/
  /*)*/
  
))

salinis said:

Next concern is how the parameters(wwid,firstname,lastname,email,role)which i passed in body  will come from external system to webap

Not sure what you mean. Line 2 parses the JSON string from the HTTP body.

salinis said:

also i need guidance how it can be authenticated

https://docs.appian.com/suite/help/24.4/Web_API_Authentication.html