Frank,
I use Yahoo mail classic on a Vista PC, but the new Yahoo mail ought to be very similar. It should be as simple as opening the "contacts" tab in Yahoo mail, then select the "Add Category" box in the row just under the tabs. Call the new category whatever your club name is, then in the "View _____" drop down box at the left end of the same row, change back to "View All Contacts." Then start checking the contacts you want to add to your new category. As soon as you get more than one selected, one of the selections in the field just to the right will be "Add to Category." Finish checking all the contacts you want to add and then click the "add to Category" box. Once you get every member contact added to your new category, you can view your new category by selecting it in the "View ____" drop down box you used to view all your contacts. Click the check box at the top of the list to check all of the members at once, then in the field to the right, select "Send email." A new message will open up with all the members of your category pre-filled. It's harder to describe than it is to do.
Good luck!