Discord is available on many platforms. For PCs it's easiest and most complex, click link and you will be forwarded either to web client or discord client if you have it installed.
For phones you need to first to download the app from the store. Launch Discord. For a first time you see dialog to join server or you see that "plus" on the left bottom. In that case, that you have it already installed. Click it and paste your link. (Like https://discord.gg/mvPpmbK
)Invite is invalid?
Ask for a new invite, some invites are set for forever, others can be time limited (Strangely it's either Unlimited, One day or shorter.). Also there are other limits, link can be changed or you joined too many servers.
How to sent PM?
Click on friend or another server member, from the text or from the right, do right click and hit "Message". This must be first allowed within the server.
Or peek left up and use "Direct Message", assuming you know player's ID. These are like "Player#1111". You can have 15 friends.
How I will see that someone sent me a message?
Peek left up again, there will be hanging number of your PMs
What about bots?
They have their own commands, check docs. You can do test with, them translate, make quizzies, interactive forms etc.. You can add them by invite too.
Discords:
