Solution:
as per this developerforce.com forum entry:
Trigger
trigger GroupReadOnly on FeedItem (before insert) {
CollaborationGroup gp = [Select OwnerId, Id From CollaborationGroup Where Name = 'Group_ReadOnly'];
List<FeedItem> feedItems = new List<FeedItem>();
for(FeedItem item : trigger.new){
if(item.ParentId == gp.Id)
{
feedItems.add(item);
}
}
if(feedItems.size() >0) GroupReadOnlyClass.FilterFeedItems(feedItems);
}
Class
public class GroupReadOnlyClass{
public static void FilterFeedItems(List<FeedItem> feedItems){
CollaborationGroup gp = [Select OwnerId, Id From CollaborationGroup Where Name = 'Group_ReadOnly'];
for(FeedItem item :feedItems){
if(item.ParentId == gp.Id)
{
if(UserInfo.getUserId()!= gp.OwnerId){
item.addError('You cannot post! Just Owner can post in this group');
}
}
}
}
}