How exactly would one say ustedes in an informal way? I'm not talking about vosotros, since it is only used in Spain.
Since for the second person plural actually there are not other pronouns different to ustedes and vosotros and vosotros is not used in many places outside Spain then ustedes is used both for formal and informal conversations.
For the singular there are other forms like «tu», «vos» and «usted» that are used differently for formal or informal situations but that does not happen for the plural where only «ustedes» is used.
As already said in @pablod76's answer there are other ways to refer to groups of people like «los señores» that may be more formal but when using the pronoun there is no problem in using ustedes either in formal or informal conversations because there is no other way.
The question being about the informal use, the answer would be there is no special way of using ustedes but if instead the situation requieres to show respect and be formal then you should show that by adding other things tu your sentence. i.e "Respetuosamente les pido a ustedes..." or "ustedes me disculparán..."