Pass list value to view model class variable in ASP.NET MVC

I would have done it a little different. You want to return a single view model to the view consisting of a list of users and a list of groups (please correct me if I am wrong). I would have initialised the lists to empty (containing no items) in the constructor of the view model:

public class UserGroup_ViewlModel
{
     public UserGroup_ViewlModel()
     {
          Users = new List<User>();
          Groups = new List<Group>();
     }

     public List<User> Users { get; set; }

     public List<Group> Groups { get; set; }
}

Then in your service layer you contruct the view model and populate the users and groups, and then return the view model to the calling method, in this case I would assume the controller's action method:

public class UserManagement_Services
{
     public UserGroup_ViewModel GetUserAndGroups()
     {
          UserGroup_ViewModel _UserAndGroupModel = new UserGroup_ViewModel();

          using(var _uow = new UserManagement_UnitOfWork())
          {
              _UserAndGroupModel.Users = _uow.User_Repository.GetAll();
              _UserAndGroupModel.Groups = _uow.Group_Repository.GetAll();
          }

          return _UserAndGroupModel;
     }
}

Just make sure that User_Repository.GetAll() and Group_Repository.GetAll() have a return type of List<User> and List<Group>.

And then your controller might look something like this:

public class UserController : Controller
{
     private IUserManagement_Services userManagementService;

     public UserController(IUserManagement_Services userManagementService)
     {
          this.userManagementService = userManagementService;
     }

     public ActionResult Index()
     {
          // The service will return a view model already populated with the users and groups
          UserGroup_ViewlModel model = userManagementService.GetUserAndGroups();

          return View(model);
     }
}

In your view you would have the following, accept the view model passed in by the action method and display the users and groups:

@model MyProject.Models.UserGroup_ViewlModel

 <table>
      <thead>
           <tr>
                <th>First Name</th>
                <th>Last Name</th>
                <th>Groups</th>
           </tr>
      </thead>
      <tbody>
           @foreach (var user in Model.Users)
           {
                <tr>
                     <td>@user.FirstName</td>
                     <td>@user.LastName</td>
                     <td>
                          <select>
                               @foreach (var group in Model.Groups)
                               {
                                    <option>group.GroupName</option>
                               }
                          </select>
                     </td>
                </tr>
           }
      </tbody>
 </table>

The code above has not been tested at all, there might be a couple of issues with the @ symbol in the view. I can see what you are trying to get at. A list of users in a table, then per line you want a dropdown list of user groups that you want to select and then send back to the controller. That is out of scope for this post, start a new one. I have helped you on the correct track, you just need to spend some more time reading up on ASP.NET MVC.

I hope this helps :)