WishList Member uses the WordPress User database to create members. So WordPress does assign each user (who then becomes a member) a WordPress User ID.
You can see the User ID number for each member if you hover your mouse over the WordPress Profile icon for a member in the Members > Manage section in WishList Member. (The square icon will appear on the far right when your mouse hovers over the row of the member)
You will see the User ID number in the string right after user_id=
Example: https://wlptest.com/wp-admin/user-edit.php?user_id=8 (In this example, the User ID is 8)
You can also see that User ID number if you click the WordPress Profile icon and view their Profile. The User ID appears at the end of the URL in the address bar.