Mentioning multiple users with Bot Framework

amatthews-yakchat - We tried this with your code as well and its working, could you also please try by using user principal names once. We hardcoded the values into SubscriptionMentions object. Also please note that we MentionStringBuilder.ToString() directly at the end.

Could you please give a try like this?

            var SubscriptionMentions = new string[]
            {
                @"{
                    Id: 'admin@M365x654992.onmicrosoft.com',
                    Name: 'Admin'
                }",
                @"{
                    Id: 'adelev@M365x654992.onmicrosoft.com',
                    Name: 'Adele'
                }",
                @"{
                    Id: 'AlexW@M365x654992.OnMicrosoft.com',
                    Name: 'Alex'
                }",
            };
            IMessageActivity OutboundMessage = Activity.CreateMessageActivity();
            OutboundMessage.Entities = new List<Entity>();
            StringBuilder MentionStringBuilder = new StringBuilder(2048);
            foreach (String MentionData in SubscriptionMentions)
            {
                ChannelAccount MentionAccount = JsonConvert.DeserializeObject<ChannelAccount>(MentionData);
                String MentionName = XmlConvert.EncodeName(MentionAccount.Name);
                if (MentionName.Contains("_x0020_"))
                {
                    MentionName = MentionName.Replace("_x0020_", " ");
                }
                Mention UserMention = new Mention()
                {
                    Mentioned = MentionAccount,
                    Text = $"<at>{MentionName}</at>"
                };
                OutboundMessage.Entities.Add(UserMention);
                MentionStringBuilder.Append($" <at>{MentionName}</at>,");
            }
            
            MentionStringBuilder.Length--;
           
            OutboundMessage.Text = MentionStringBuilder.ToString();
            await turnContext.SendActivityAsync(OutboundMessage, cancellationToken);