How do you say an event will be casual?
In Chinese, "随便" has a lot of meanings.


It can be used to represent words ending with '-ever', like whatever, whenever, like what you thought.
It can be used also as an adjective, and in this way it means 'don't care much about details'
It can be used as an adverb, which means 'freely' or 'do as one's will'


But I think maybe "随便" is not very suitable in your example. "不正式", which means not formal, or "随意", which is used to describe someone or something without many constraints, may be more appropriate. But actually, they don't differ a lot :)