Well, it's not really a "Wii Party U Server", but it's a place where you can talk about Wii Party U stuff.

Here's a place to talk about Wii Party U stuff! This is the Wii Series Speedrunning discord server.

https://discordapp.com/invite/FKjCMnx

When you join the server, you won't have access to any game specific channels. Type !addrole-wiiparty in the #bot channel. This will give you access to the Wii Party and Wii Party U channels! You can also get access to other game's channels, just look in the #welcome channel!