How do i implement custom permissions?

Question

I want to do the following using sharepoint 2010 permissions:

There are 4 groups with multiple members, each member can only add/edit their own list items and list items belonging to members of their group. They can view items of any group.

If the user is in Group A they should be able to add/edit all 4 group members list items.

How would i go about doing this?

Answer 1

I dont think there is simple OOB way of doing this, but it's achievable if you use event receiver, you need to handle both ItemUpdating and ItemDeleting, in the event receiver check the current users groups and authors group by SPUser.Groups, if current user is in Group A allow the operation otherwise check current user and author is in same group - if they are then allow the operation, if they are not cancel the operation, should be something like -

properties.ErrorMessage = "Permission Denied";
properties.Status = SPEventReceiverStatus.CancelWithError;
properties.Cancel = true;

Answer 2

1. put the members of group a into b c d.
2. assign the same permissions to the list/items with group b c d
3. i don't know...