First click the 3-Dots and set your name so that people know who you are!
Click the contact you wish to start a chat with from the list of contacts on the left hand side
Click the groups tab, then the group you wish to visit to start chatting with the group.
Select a contact then click the ^ arrow next to the chat box. Here you can click the Broadcast Icon.
Sharing a file
Select a contact then click the paperclip icon next to the chat box.