public class GridGroupPanelCommandContext :
    GridCommandContextBase,
    IGridCommandContext,
    ICommandContextBase

@inject WeatherForecastService ForecastService

<DxGrid Data="@forecasts"
        ShowGroupPanel="true"
        ContextMenus="GridContextMenus.GroupPanel"
        CustomizeContextMenu="OnCustomizeContextMenu">
    <Columns>
        <DxGridSelectionColumn />
        <DxGridDataColumn FieldName=@nameof(WeatherForecast.TemperatureC) Caption="Temp. (C)" />
        <DxGridDataColumn FieldName=@nameof(WeatherForecast.TemperatureF) Caption="Temp. (F)" />
        <DxGridDataColumn FieldName=@nameof(WeatherForecast.Summary) Caption="Summary" GroupIndex="0" />
        <DxGridDataColumn FieldName=@nameof(WeatherForecast.Date) DisplayFormat="dd/MM/yyyy" />
    </Columns>
    <GroupSummary>
        <DxGridSummaryItem SummaryType="GridSummaryItemType.Count" FieldName="Date" FooterColumnName="Date" />
    </GroupSummary>
</DxGrid>

@code {
    private List<WeatherForecast>? forecasts;

    protected override async Task OnInitializedAsync() {
        forecasts = await ForecastService.GetForecastAsync();
    }
    void OnCustomizeContextMenu(GridCustomizeContextMenuEventArgs args) {
        if (args.Context is GridGroupPanelCommandContext groupPanelContext) {
            var isGroupedColumnsVisible = groupPanelContext.Grid.ShowGroupedColumns;
            string itemText = isGroupedColumnsVisible ? "Hide Grouped Columns" : "Show Grouped Columns";
            args.Items.AddCustomItem(0, itemText, () => {
                groupPanelContext.Grid.BeginUpdate();
                groupPanelContext.Grid.ShowGroupedColumns = !isGroupedColumnsVisible;
                groupPanelContext.Grid.EndUpdate();
            });
        }
    }
}

using System;

public class WeatherForecast {
    public int ID { get; set; }
    public DateTime Date { get; set; }
    public int TemperatureC { get; set; }
    public int TemperatureF => 32 + (int)(TemperatureC / 0.5556);
    public string? Summary { get; set; }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

public class WeatherForecastService {
    public readonly string[] Summaries = new[] {
        "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
    };
    private List<WeatherForecast>? Forecasts;
    public Task<List<WeatherForecast>> GetForecastAsync() {
        if (Forecasts == null) {
            var rnd = new Random();
            Forecasts = Enumerable.Range(1, 50).Select(index => new WeatherForecast {
                ID = index,
                Date = DateTime.Today.AddDays(index),
                TemperatureC = rnd.Next(-20, 55),
                Summary = Summaries[rnd.Next(Summaries.Length)]
            }).ToList();
        }
        return Task.FromResult(Forecasts);
    }
    public Task<List<WeatherForecast>> ChangeForecastAsync(WeatherForecast changed) {
        var originalForecast = Forecasts.FirstOrDefault(i => i.ID == changed.ID);
        if (originalForecast != null) {
            originalForecast.TemperatureC = changed.TemperatureC;
            originalForecast.Summary = changed.Summary;
            originalForecast.Date = changed.Date;
        } else {
            Forecasts.Add(changed);
        }
        return Task.FromResult(Forecasts);
    }
    public Task<List<WeatherForecast>> Remove(WeatherForecast forecast) {
        Forecasts.Remove(forecast);
        return Task.FromResult(Forecasts);
    }
}

// ...
builder.Services.AddSingleton<WeatherForecastService>();