It has really been a head scratching task for me. I 've tried many options but nothing worked. Finally I found a workaround on google to achive this by TaskScheduler.

PROBLEM
When we run Teamsite user administration command line tool IWUSERADM.exe though ASP.Net it gives following error:

Application popup: cmd.exe - Application Error : The application failed to initialize properly (0xc0000142). Click on OK to terminate the application.

CAUSE
No specific cause, it seems to be a bug, supposed to be resolved with this Microsoft patch http://support.microsoft.com/kb/960266. and there is nothing related to permission issue, y web application is impersonated with an administrator account. off course running a bat file from dmin account is a potential secury threat but for this scenario lets conifned our discussion to run the command line tool.

RESOLUTION
I have not tried this patch as I have not permitted to run this patch on server. Below are the steps to achive the requirement.

1/ Create a batch file which runs the IWUSERADM.exe. 
       echo - Add Teamsite User
   CD E:\Appli\GN00\iw-home\bin
   iwuseradm add-user %1

2/ Temporarily create a schedule task and run  the .bat file by scheduled task by ASP.Net code using TaskScheduler http://www.codeproject.com/KB/cs/tsnewlib.aspx.

3/ Here is the function:
private int AddTeamsiteUser(string strBatchFilePath, string strUser)
{
//Get a ScheduledTasks object for the local computer.
ScheduledTasks st = new ScheduledTasks();
// Create a task
Task t;
try{
t = st.CreateTask("~AddTeamsiteUser");
}
catch
{
throw new Exception("Schedule Task ~AddTeamsiteUser already exist.");
}

return t.ExitCode;

Below are few resources related to the above scenario:-
- Task Scheduler Class Library for .NET
 http://www.codeproject.com/KB/cs/tsnewlib.aspx
- Run a .BAT file from ASP.NET
 http://codebetter.com/blogs/brendan.tompkins/archive/2004/05/13/13484.aspx
- TaskScheduler Class
 http://msdn.microsoft.com/en-us/library/system.threading.tasks.taskscheduler.aspx
- Application Hangs whle running iwuseradm.exe through ASP.Net
 http://bytes.com/topic/asp-net/answers/733098-system-diagnostics-process-hangs
 

t.ApplicationName = strBatchFilePath;
t.Parameters = strUser;
t.Comment = "Adding user to Teamsite Application"