One new thing to get used to with discourse is the way groups and categories work with each other.
Content lives in categories, people live in groups.
And you set permissions to categories based on groups.
So if you have a mailing list or google group you want to move over to discourse you’d want to set up a group for the user list and a category for the messages. If you want to have 10 lists you are moving to one discourse site, you’d create 10 groups and 10 categories…
… though my experience has been that discourse is different enough that you may want to think about setting up one discourse install with multiple categories for each group you are migrating over. 