Link your Minecraft account to your Discord account using this tutorial video:
Proximity Chat
Join the Prox-Chat-Lobby voice channel whenever you play the server After completing these two steps, whenever you enter proximity voice chat range of another player(s) using proximity chat, you will be put into a voice channel with them
Server Chat
You can use#server-chat in the discord to communicate with people on the server. Just type whatever you want into the channel, and everyone on the server will see it and be able to respond to you.