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 view the WordPress User ID if you go to the Members tab in WishList Member and hover your mouse over a Member Username.
A line of text will appear near the bottom left of your browser screen which includes the following: user_id=
The number that appears within that line of text is the WordPress User ID for the member. WordPress assigns this number to each user/member.