ご is particle to make honorific expression. Normally ご and お add to just before verb and adjective.(There are some exceptions)If you would like to say `` I will join`` , no need to add ご、just 参加します。If your boss or teacher respectable person will join, ご参加されます。This is the same expression as passive expression.To use correct honorific expression is very hard even for Japanese.

I think it should be 参加します・参加いたします。

「ご参加します」という表現は不自然です。

konnichiwa, nihongo wa ikura des ka?