The easest way to tell all the members about a last minute event, climbing day/walk etc, is as follows,
1) Add and save the event in the normal manner.
2) You will see within the text of your event the words "INVITE MORE PEOPLE". Click into this
3) Click the heading "INVITE ALL MEMBERS", ignore the rest of the page.
4) There is then space to add an extra message if you wish but it isn't necessary.
5) click send
6) This message will be immediately sent to every members email address
Hope this helps.