diff --git a/SharepointToolbox.Tests/Services/Export/HtmlExportServiceTests.cs b/SharepointToolbox.Tests/Services/Export/HtmlExportServiceTests.cs
index ac8de01..2889bf6 100644
--- a/SharepointToolbox.Tests/Services/Export/HtmlExportServiceTests.cs
+++ b/SharepointToolbox.Tests/Services/Export/HtmlExportServiceTests.cs
@@ -5,15 +5,14 @@ namespace SharepointToolbox.Tests.Services.Export;
 
 /// <summary>
 /// Tests for PERM-06: HTML export output.
-/// These tests reference HtmlExportService which will be implemented in Plan 03.
-/// Until Plan 03 runs they will fail to compile — that is expected.
 /// </summary>
 public class HtmlExportServiceTests
 {
     private static PermissionEntry MakeEntry(
         string users, string userLogins,
-        string url = "https://contoso.sharepoint.com/sites/A") =>
-        new("Web", "Site A", url, true, users, userLogins, "Read", "Direct Permissions", "User");
+        string url = "https://contoso.sharepoint.com/sites/A",
+        string principalType = "User") =>
+        new("Web", "Site A", url, true, users, userLogins, "Read", "Direct Permissions", principalType);
 
     private static ReportBranding MakeBranding(bool msp = true, bool client = false)
     {
@@ -22,6 +21,13 @@ public class HtmlExportServiceTests
         return new ReportBranding(mspLogo, clientLogo);
     }
 
+    private static IReadOnlyDictionary<string, IReadOnlyList<ResolvedMember>> MakeGroupMembers(
+        string groupName, params ResolvedMember[] members) =>
+        new Dictionary<string, IReadOnlyList<ResolvedMember>>(StringComparer.OrdinalIgnoreCase)
+        {
+            [groupName] = members.ToList()
+        };
+
     [Fact]
     public void BuildHtml_WithKnownEntries_ContainsUserNames()
     {
@@ -87,4 +93,79 @@ public class HtmlExportServiceTests
         Assert.Contains("data:image/png;base64,bXNw", html);
         Assert.Contains("data:image/jpeg;base64,Y2xpZW50", html);
     }
+
+    // ── Group expansion tests (Phase 17) ──────────────────────────────────────
+
+    [Fact]
+    public void BuildHtml_NoGroupMembers_IdenticalToDefault()
+    {
+        var entry = MakeEntry("Site Members", "i:0#.f|membership|group@contoso.com", principalType: "SharePointGroup");
+        var svc = new HtmlExportService();
+        var htmlDefault = svc.BuildHtml(new[] { entry });
+        var htmlNullNull = svc.BuildHtml(new[] { entry }, null, null);
+
+        Assert.Equal(htmlDefault, htmlNullNull);
+    }
+
+    [Fact]
+    public void BuildHtml_WithGroupMembers_RendersExpandablePill()
+    {
+        var entry = MakeEntry("Site Members", "i:0#.f|membership|group@contoso.com", principalType: "SharePointGroup");
+        var groupMembers = MakeGroupMembers("Site Members", new ResolvedMember("Alice", "alice@co.com"));
+        var svc = new HtmlExportService();
+        var html = svc.BuildHtml(new[] { entry }, null, groupMembers);
+
+        Assert.Contains("onclick=\"toggleGroup('grpmem0')\"", html);
+        Assert.Contains("class=\"user-pill group-expandable\"", html);
+    }
+
+    [Fact]
+    public void BuildHtml_WithGroupMembers_RendersHiddenMemberSubRow()
+    {
+        var entry = MakeEntry("Site Members", "i:0#.f|membership|group@contoso.com", principalType: "SharePointGroup");
+        var groupMembers = MakeGroupMembers("Site Members", new ResolvedMember("Alice", "alice@co.com"));
+        var svc = new HtmlExportService();
+        var html = svc.BuildHtml(new[] { entry }, null, groupMembers);
+
+        Assert.Contains("data-group=\"grpmem0\"", html);
+        Assert.Contains("display:none", html);
+        Assert.Contains("Alice", html);
+        Assert.Contains("alice@co.com", html);
+    }
+
+    [Fact]
+    public void BuildHtml_WithEmptyMemberList_RendersMembersUnavailable()
+    {
+        var entry = MakeEntry("Site Members", "i:0#.f|membership|group@contoso.com", principalType: "SharePointGroup");
+        var groupMembers = MakeGroupMembers("Site Members"); // empty list
+        var svc = new HtmlExportService();
+        var html = svc.BuildHtml(new[] { entry }, null, groupMembers);
+
+        Assert.Contains("members unavailable", html);
+    }
+
+    [Fact]
+    public void BuildHtml_ContainsToggleGroupJs()
+    {
+        var entry = MakeEntry("Site Members", "i:0#.f|membership|group@contoso.com", principalType: "SharePointGroup");
+        var groupMembers = MakeGroupMembers("Site Members", new ResolvedMember("Alice", "alice@co.com"));
+        var svc = new HtmlExportService();
+        var html = svc.BuildHtml(new[] { entry }, null, groupMembers);
+
+        Assert.Contains("function toggleGroup", html);
+    }
+
+    [Fact]
+    public void BuildHtml_Simplified_WithGroupMembers_RendersExpandablePill()
+    {
+        var innerEntry = new PermissionEntry("Web", "Site A", "https://contoso.sharepoint.com/sites/A",
+            true, "Site Members", "i:0#.f|membership|group@contoso.com", "Read", "Direct Permissions", "SharePointGroup");
+        var simplifiedEntry = new SimplifiedPermissionEntry(innerEntry);
+        var groupMembers = MakeGroupMembers("Site Members", new ResolvedMember("Bob", "bob@co.com"));
+        var svc = new HtmlExportService();
+        var html = svc.BuildHtml(new[] { simplifiedEntry }, null, groupMembers);
+
+        Assert.Contains("onclick=\"toggleGroup('grpmem0')\"", html);
+        Assert.Contains("class=\"user-pill group-expandable\"", html);
+    }
 }