I can’t wait to express my cordiallywelcome to you for the conference to be held at the meeting room, at which youare invite to be a guest speaker.

有人给看下嘛

错误太多。
先写出中文，
别人帮你翻

就at which 这个用法来说，没什么问题，其实这里用in which个人觉得也可以，还有这句句子的前后逻辑有点奇怪，个人觉得这样说会更好点：i cant wait to express my gratitude to you for agreeing to attend this meeting which will be held at the conference room where you are invited as a guest speaker，这里where和at which或in which 可以互换，我个人选择用where因为前面已经出现了个which，避免重复

我会改为：
I can't wait to extend my cordial welcome to your coming, as a guest speaker, to the conference held at the meeting room.