I'll explain this with an example, let's call it 'Best of'. You have a group with all your pictures of the summer holidays, far too much to show to your friends (some boring ones in between, but you like to keep them).
You create another group, which is a link to the group mentioned before. To do this, select the original group, open up the context menu in the group the new one should be in and select Link marked into this group.
The new group does not contain any pictures yet, but it shares the comment and keywords with the original group. So when you edit the comment or keywords on one of them, the change takes effect for both of them.
Inside of the the new group you create links to the original pictures you want to show to your friends by selecting them in the original group, opening up the context menu in the new group and selecting Link marked into this group.
Again, if you edit comment/keywords/date/time on the link or on the original picture, it takes effect on both of them.
To link an item from a comment, the destination item needs to have an ID and you need to know this ID. So open the properties dialog of the destination item to look it up. If there is no ID yet, hit the button Create ID which assigns an ID to the item.
Lets assume the destination item's ID is 42. You can link it by writing <ID=42> into a comment.
You can also link external files, which contain a long description for example. Lets assume it is in holidays2005.htm. To link this file, simply write <FILE=holidays2005.htm> into a comment.
Until now it is not clear in which directory this file is in. The answer is simple: in one of the directories where picture files of the same group are.
If this link is in the comment of a picture, put the file into the directory where the picture is.
If this link is in the comment of a group, put the file into a directory where pictures of the group are.