http://xyz.sharepoint.com/sites/MainSite/  - SiteUrl

  http://xyz.sharepoint.com/sites/MainSite/SampleSite1 - Subsite1's Url
  http://xyz.sharepoint.com/sites/MainSite/SampleSite2 - Subsite2's Url

SecureString password = new SecureString();
string pwd = "Pass123";
string UserName = "abc@xyz.com";
password = convertToSecureString(pwd);
ClientContext clientContext = new  ClientContext("http://xyz.sharepoint.com/sites/MainSite/");
clientContext.Credentials = new SharePointOnlineCredentials(UserName, password);

public void AddUserToDMSite(string useremail, string securityGroupName)
        {
     GroupCollection collGroup = SPContext.Web.SiteGroups;
     Group oGroup1 = collGroup.GetByName("UserList");
     Group oGroup2 = collGroup.GetByName(securityGroupName);
     UserCollection oUserCollection1 = oGroup1.Users;
     UserCollection oUserCollection2 = oGroup2.Users;
     SPContext.Load(oUserCollection1);
     SPContext.Load(oUserCollection2);
     SPContext.ExecuteQuery();
     var uname = oGroup1.Users.GetByEmail(useremail);
     var userCheck = oUserCollection2.Where(u => u.Email == useremail).FirstOrDefault();
     if (userCheck == null)
     {
          Microsoft.SharePoint.Client.User oUser2 = oGroup2.Users.AddUser(uname);
     }
     SPContext.ExecuteQuery();
     }