Random users always showing same 8 users

Actually, rand is not a valid argument for orderby in the WP_User_Query class (it is in the WP-Query class). So, it should default to user_login, giving you an alphabetical list. Also, you should get all users that fit the member and exclude criteria from this query, not just eight.

This suggests there is other code interfering with yours.

My suggestion would be to start with adding 'number' => 100 to $args. That should overrule any other code limiting the amount of results. You could then use shuffle to randomize the array returned or pick some random elements from the array with array_rand.