Box 1: Yes -
The ActivityHandler.OnMembersAddedAsync method overrides this in a derived class to provide logic for when members other than the bot join the conversation, such as your bot's welcome logic.
Box 2: Yes -
membersAdded is a list of all the members added to the conversation, as described by the conversation update activity.
Box 3: No -
Reference:
https://docs.microsoft.com/en-us/dotnet/api/microsoft.bot.builder.activityhandler.onmembersaddedasync?view=botbuilder-dotnet-stable
