Nintex Workflow has actions “Web Request” and “Call HTTP Web Service” which can be used to make calls to the REST API of SharePoint Online/2013 to automate creation of SharePoint groups in a site collection.

Add the following steps to the workflow

1. Create a Text variable GroupName and Use “Set Workflow Variable” to assign it to a value.
2. Get request digest from targeted site collection. Please note that workflow app permissions need to amended to give site collection full control access.        
   • Add “App Step” action
   • Add “Web Request” action. Fill in the following fields
     • URL: <siteURL> /_api/ContextInfo
     • Method : POST
     • Content type : application/x-www-form-urlencoded
     • Header Name (Key) : Accept     Header value: application/json;odata=verbose

       

     • Body : Choose “Content” option  and enter “{}” in text field
     • UserName: Credentials who has access to targeted site
     • Password: password of above UserName
     • Store response content in : Create a variable “digest token”
   • Add “Query XML” action. Fill in the following fields
     • XML source: Choose “Content” and pick variable “digest token”
     • XPath query: /d:GetContextWebInformation/d:FormDigestValue
     • Return result as :Text
     • Query result in : create a text variable strDigestInfo
   • Add “Log to History” action. Print the strDigestInfo to make sure valid request digest are used  
3. Create SharePoint Group using REST API ‍/_api/web/sitegroups  
   • Add “App Step” action. Optionally add “Log to History List” action to log status of workflow.
   • Add “Build Dictionary” action to build “Request Headers”.
     •  Add the following key value pair as Text- content-type:  application/json; odata=verbose
       – X-RequestDigest : {Variable:strDigestInfo}
     • Save Output into RequestHeaders
   • Add “Build Dictionary” action to build “Metadata”
     •  Add the following key/value pair- key: type    Type:Text  Value: SP.Group
       
   • Add “Build Dictionary” action to build “Parameters”
     •  Add the following key/value pair- Key: __metadata   Type:Dictionary   Value: Workflow Variable GroupMetadata
       – Key: Description  Type: Text  Value: Group created automatically from Nintex workflow
       – Key: Title  Type:Text  Value:{Variable:GroupName}
     • Assign Output to a dictionary variable GroupParameters
       
   • Add “HTTP Web Service”action to create group
     • Fill in Address : <SiteURL>/_api/web/sitegroups
     • Set Request Type to “HTTP Post”
     • Set Request Headers to dictionary variable RequestHeaders
     • Set Request Content to dictionary variable GroupParameters
     • Set Response Headers to new dictionary variable GroupResponseHeaders
     • Set Response Content to new dictionary variable GroupResponseContent
     • Set Response Status Code to new text variable GroupResponseStatusCode
   • Log to History list the response
4. Retrieve SharePoint Group using REST API ‍ ‍/_api/web/sitegroups/getbyname 
   • Add “Add Step” action
   • Optionally add “Log History List” action to log workflow status.
   • Add a “Web Request” action
     • Set URL to <SiteURL>/_api/web/sitegroups/getbyname(‘‍{Variable:GroupName}‍’)?$select=Id
     • Choose  Get for Method
     • Click Add Header
     • Set Header Key to “Accept”
     • Set Header Value to “application/json;odata=verbose”
     • Set UserName to user who has access to site collection
     • Set Password to password of specified user
     • Set Store response content in a Text variable GroupRequestIDXMLAdd
   • Add “Log History List” action to log response GroupRequestIDXMLAdd from web request. The response is in JSON format. Nintex does not have a JSON parser action to retrieve value of property. Use “Regular Expression” action as described in the next step is used to retrieve Id value
   • Add “Regular Expression” action
     • Set String to ‍{Variable:GroupRequestIDXML}
     • Set String Operation to Extract
     • Set pattern to (?<=”Id”:).*?(?=})
     • Save Output to text variable strGroupIDCol
   • Add “Get Item from Collection” action.
     • Set Target Collection to variable strGroupIDCol
     • Set Index to 0
     • Set Output to SPGroupId
   •  Add “Log History List” action to log strGroupIDCol
5. Add Contribute Role to new Group
   • Add App Step action  
   • Add “Call HTTP Web Service” Action 
     • Set URL to <siteURL>‍/_api/web/roleassignments/addroleassignment(principalid=‍{Variable:SPGroupId}‍,roleDefId=1073741827)
     • Set Request Type to “HTTP Post”
     • Set Request Headers to dictionary “RequestHeaders”
     • Set Response Headers to new dictionary variable RoleAssignResponseHeaders
     • Set Response Content to new dictionary variable RoleAssignmResponseContent
     • Set Response Status Code to new text variable RoleAssignmResponseStatusCode
   • Add “Log to History List” action to log response of web request

Save and Publish the workflow. If the workflow runs succesfully