And that's about it. You've created a group with a custom icon in the Google Messages app. Similarly, you can change the icon of an already available group. To do this, simply tap the group name ...

Google Messages is testing custom RCS group chat icons. Users will be able to set their own chosen icon instead of defaulting to a grid made up of users’ initials or individual profile pictures.

Custom group icons are not yet widely available on devices running the Messages beta that we checked today. It will be a nice quality-of-life improvement once rolled out.