The shortest and neatest way to write it is "Anyone interested may attend."

Any interested person can attend this lecture. or Anyone having interest in this lecture can attend it. or If anyone is interested he/she or they can attend this lecture. I apologize if I am wrong but I guess we cannot say like this " If anybody is interested, you can attend the lecture." In such case it should be like this " if you are interested, you can attend the lecture"

Anyone who is interested in this topic can attend the lecture .