Roblox on Xbox has an integrated voice chat system that allows you to chat with your friends and party members.
If a microphone is connected via a headset or Kinect, you are automatically placed into voice chat with other players in the experience you joined.
Note: Voice communication must be enabled in Xbox Settings.
Xbox Live Party Chat
Roblox also allows for the use of Xbox Live Party Chat with your Xbox Live friends. When active, this feature automatically overrides in-experience chat.
Note: You can switch to in-experience chat by going to the "Your party" panel in Xbox and selecting "Switch to experience chat".
How to start a party on Xbox One