When you have finished recording and save your Blrt the 'Send conversation' window will appear giving you the option to 'add people' to your Blrt via Email or Facebook or 'share a link' to your Blrt via text message, Facebook message, Twitter, or email. You are also able to copy and paste the link to your Blrt.
If you need to add more people later you may do so from your 'Conversations' or 'Archive' screens. Simply select the Blrt you want to add people to, then select the green 'Add' button at the bottom of the screen. This will open the 'Send conversation' window.
You can also add people by selecting the Blrt conversation you want to add them to and Tapping the icon '...' (triple dot) icon at the top right of the screen. This will open your conversation options where you can select the 'Add people' tab that will open up the 'Send conversation' window.