search on steemit of the word discord and see if people link to their channels or post a invite. make sure you READ FIRST what their group is about. it doesn't make sense to join them randomly
two groups that might interest you are:
ADSactly: https://discord.gg/uDWVd3
Minnow-Power: https://discord.gg/kZqrtt <- this also has the GinaBot that tells you everytime you get mentioned, upvoted or resteemed so you can act on it.