Get the list of all users from Sharepoint Online

You could utilize Web.SiteUsers property to gets the all users that belong to the site collection. But since it returns all the users including system accounts, the following examples demonstrate how to return only real users.

CSOM

using (var ctx = GetContext(webUri,userName,password)) { var users = ctx.LoadQuery(ctx.Web.SiteUsers.Where(u => u.PrincipalType == PrincipalType.User && u.UserId.NameIdIssuer == "urn:federation:microsoftonline")); ctx.ExecuteQuery(); } 

where

public static ClientContext GetContext(Uri webUri, string userName, string password) { var securePassword = new SecureString(); foreach (var ch in password) securePassword.AppendChar(ch); return new ClientContext(webUri) {Credentials = new SharePointOnlineCredentials(userName, securePassword)}; } 

REST

Endpoint: /_api/web/siteusers?$filter=PrincipalType eq 1 and UserId/NameIdIssuer eq 'urn:federation:microsoftonline' Method: GET 

In the client object model, the list groups in a site-collection needs to be fetched first and then based on the groups another call needs to be made to SharePoint Online to get the list of users. The following code snippet explains it all.

namespace GetUsersInGroupCSOM { class Program { static void Main(string[] args) { //Replace it with the url of your tenant or your site-collection string SiteUrl = "https://yoursite.sharepoint.com"; System.Uri oUri = new System.Uri(SiteUrl); using (ClientContext oClientContext = new ClientContext(SiteUrl)) { //Replace it with your user id for SharePoint Online string UserName = "[email protected]"; //Replace it with your password string Password = "yourpassword"; //Create a SecureString object from password string, needed for SharePointOnlineCredentials class SecureString SecurePassword = GetSecureString(Password); oClientContext.Credentials = new SharePointOnlineCredentials(UserName, SecurePassword); //Load the site-collection groups using CSOM oClientContext.Load(oClientContext.Web.SiteGroups); oClientContext.ExecuteQuery(); GroupCollection oSiteCollectionGroups= oClientContext.Web.SiteGroups; Console.WriteLine("List of groups in the site collection"); Console.WriteLine("-------------------------------------"); foreach (Group oGroup in oSiteCollectionGroups) { Console.WriteLine(oGroup.Title); Console.WriteLine("\n"); } //Load the users collection in the Group 1 oClientContext.Load(oSiteCollectionGroups[1].Users); oClientContext.ExecuteQuery(); Console.WriteLine("List of users in the first group of site-collection"); Console.WriteLine("-------------------------------------"); foreach(User oUser in oSiteCollectionGroups[1].Users) { Console.WriteLine(oUser.Title); Console.WriteLine("\n"); } Console.ReadLine(); } } private static SecureString GetSecureString(String Password) { SecureString oSecurePassword = new SecureString(); foreach (Char c in Password.ToCharArray()) { oSecurePassword.AppendChar(c); } return oSecurePassword; } } } 

Source: How to get Users and Groups in SharePoint 2013 Online using CSOM

Also check this one:

Office 365 - Retrieve User Profile Properties using CSOM with PowerShell