i can’t request a new user without specifying a Remove existing user

@Nbibi .....could you please provide more details on what exactly you're doing?

@kunalpatel ,

There are a number of controls on the form that either pulls their data from a SharePoint list or call the Workflow Web API for additional processing like. 

Calls the GenerateUsernameAndReturnPotentialDuplicates web method in the Workflow Web API to return potential duplicate AD accounts based on the entered First Name and Last Name.

Calls the GenerateUsernameAndReturnPotentialDuplicates web method in the Workflow Web API to determine whether there are potential duplicate AD accounts based on the entered First Name and Last Name. This provides a mechanism to hide the Potential Duplicates control if there are no duplicates.

the problem I am having is that I need to replace the Old user which is leaving the company and i need to replace it with a new user who is already in the AD group. 

i am using below query 

Calls the QueryADForGroups web method in the Workflow Web API to return a list of AD groups based on the following LDAP query: LDAP://OU=_ _ _ - Companies,OU=_ _ _ - system ,OU=system Security Groups,OU=system name Groups,DC=Company name,DC=net

@Nbibi ....I haven't used API that much. But I kind of had a similar requirement where I had to ask the AD group owner to review members every year. For that below is my approach:

1. AD team's list where they will submit the LDAP query
2. Workflow in the background will trigger and for each AD group, it will get the list of members and some other information and will create a list item in another list (list used by owners to review)
3. This list has Nintex forms which has two panels
   1. Existing members - It will display Name, Choice to keep or remove
   2. New members - It will have PG control to add users
4. Once they submit the form, workflow will trigger and will create following items in 3rd list
   1. List of members to keep
   2. List of members to remove
   3. List of members to add
5. In the 3rd list, workflow runs every 12 hours and will grab all the items available at that time and will create a CSV file in a document library and will delete only those items which was used in creating CSV will be deleted
6. In this library, when an item is a workflow will run and it will copy the file to AD server where a task scheduler runs the script which will be update respective AD groups

If you're confident about the information provided by your users then instead of creating CSV file, you can use update AD group action