My user list has recently grwon from a few hundred to over 20,000. As a result, I need to divide it into smaller lists for mailing. I can't see an easy way of doing this other than by adding users one by one to new lists. That will take a long time and I think there must be a better way and would be grateful for help.
Just to give detail, I had only one list before called 'newletter' and of course all users were on it. I now propose to divide the users between three lists ie, 'newsletter 1', 'newsletter 2', 'newsletter 3'. There are no criteria for allocation: the users came from a joint action with another much larger charity and the details are only name and email. So an y user can be on any list: all I want to do is divide it in three.