A friend of mine owns a minecraft realms server and I can't join. It says:Minecraft authetication servers are down for maintenance". But the weird thing is that some friends are on the server ,playing. Is that normal? Do I have to wait out?
As the comments say there are several possibilities:
You were already in, then crashed: You will need to wait for the server to kick you out, because to the server you are still logged into the server and the same player can't be logged in more then once.
There is a whitelist on the server: Your friend will need to add you to the whitelist file. For your friend to add you, he need to go to the options of his Realms, then go to the options for whitelisting people and add your username.
You have an issue with your port configuration: Try connecting to another server/Realm. If you cannot connect to any other servers, then you have an issue connecting into multiplayer. See the official forum and if you find nothing contact Mojang support, they will help you as it is a technical issue.
Note: Minecraft Realms needs an official version of Minecraft, not a pirated one.