Basically, you're getting together with a group of people who will be travelling to WDW or DL around the same time as you and splitting the cost of a Photopass CD. One person is the leader of the group and is responsible for pre-paying for the CD, collecting money from the group and then copying everyone's pictures to individual disc(s) and mailing them out.
The good thing about a share is the reduced cost of the Photopass CD but if you're the kind of person who is a little more private, you might not want other people looking at your photos. You also have to trust that the person who's running the share is organized.
Other things to keep in mind are - the photos expire 30 days from the date they're taken and then you have another 30 days from the date they're claimed on the Photopass system. So you want to make sure everyone in your share can keep to these dates.
I've organized two shares before and they turned out well