Core/RDF: fix counting issue with the achievement Looking for More (and similar)

Port From (https://github.com/TrinityCore/TrinityCore/commit/01be9f44af0687f03c34f0221dc285ec22803969)

Source/Game/DungeonFinding/LFGManager.cs

@@ -22,11 +22,11 @@ using Game.DataStorage;
 using Game.Entities;
 using Game.Groups;
 using Game.Maps;
+using Game.Networking.Packets;
 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
-using Game.Networking.Packets;
 
 namespace Game.DungeonFinding
 {
@@ -1052,6 +1052,15 @@ namespace Game.DungeonFinding
                         break;
                 }
 
+                // Store the number of players that were present in group when joining RFD, used for achievement purposes
+                Player _player = Global.ObjAccessor.FindConnectedPlayer(pguid);
+                if (_player != null)
+                {
+                    Group group = _player.GetGroup();
+                    if (group != null)
+                        PlayersStore[pguid].SetNumberOfPartyMembersAtJoin((byte)group.GetMembersCount());
+                }
+
                 SetState(pguid, LfgState.Dungeon);
             }
 
@@ -1374,7 +1383,15 @@ namespace Game.DungeonFinding
 
                 // Update achievements
                 if (dungeon.difficulty == Difficulty.Heroic)
-                    player.UpdateCriteria(CriteriaType.CompletedLFGDungeonWithStrangers, 1);
+                {
+                    byte lfdRandomPlayers = 0;
+                    byte numParty = PlayersStore[guid].GetNumberOfPartyMembersAtJoin();
+                    if (numParty != 0)
+                        lfdRandomPlayers = (byte)(5 - numParty);
+                    else
+                        lfdRandomPlayers = 4;
+                    player.UpdateCriteria(CriteriaType.CompletedLFGDungeonWithStrangers, lfdRandomPlayers);
+                }
 
                 LfgReward reward = GetRandomDungeonReward(rDungeonId, player.GetLevel());
                 if (reward == null)

Source/Game/DungeonFinding/LFGPlayerData.cs

@@ -116,6 +116,16 @@ namespace Game.DungeonFinding
             return m_SelectedDungeons;
         }
 
+        public void SetNumberOfPartyMembersAtJoin(byte count)
+        {
+            m_NumberOfPartyMembersAtJoin = count;
+        }
+
+        public byte GetNumberOfPartyMembersAtJoin()
+        {
+            return m_NumberOfPartyMembersAtJoin;
+        }
+
         // General
         RideTicket m_Ticket;
         LfgState m_State;
@@ -127,5 +137,8 @@ namespace Game.DungeonFinding
         // Queue
         LfgRoles m_Roles;
         List<uint> m_SelectedDungeons = new();
+
+        // Achievement-related
+        byte m_NumberOfPartyMembersAtJoin;
     }
 }