You may only sort on one column at a time. Look carefully at the top of the column you wish to sort and you will see two small black arrows. – Click on these and the column will be sorted in descending order, click again and the sort will be ascending click a third time and the column will be unsorted and the list will revert to membership number order.

There are other useful functions for filtering columns.  At the top of the column just under its title is the word “Contains” to the right of this is a down arrow. Click on this and there is a drop-down list of criteria that you can filter on such as “Does not contain” ” Greater Than” etc. You can filter on as many columns as you like.  One word of warning, the filter does not work on Numbers, fortunately Membership numbers are actually Text as they can contain letters, so you can filter on them.