I'm sure everyone already knows that you can chat in Minecraft, but Venture does have a few other ways to chat with other members.
To privately message a player do /msg <player> <message>. Only you and the player you are messaging can read these messages. This only works for online players.
Messaging offline players can be down with sending a mail. Do /mail send <player> <message> to send a message to a player. To read any mails you've received do /mail read, to delete all your mails to /mail clear.
You can toggle party chat with /party chat. Once you are in a party, all party members online can read the messages you send.