When dividing (or crop masking) objects, you have to use one path.
This can be in the form of many objects that have been joined together as a compound path (especially good if your masking shape resembles a doughnut or letter forms, etc) or as shapes that have been merged together (such as a circle overlapping a rectangle).
Try ungrouping the upper most object and merging the shapes together with a compound path (or simple "merge")... you should then be able to mask/divide any objects below it, etc.
Groups are not appropriate for dividing/cropping as illustrator doesn't know how to treat the overlapping shapes (in which order) when adding a new shape to the mix.
If you explain what you mean by "united" shape, I can probably explain it a bit more?
When dividing (or crop masking) objects, you have to use one path.
This can be in the form of many objects that have been joined together as a compound path (especially good if your masking shape resembles a doughnut or letter forms, etc) or as shapes that have been merged together (such as a circle overlapping a rectangle).
Try ungrouping the upper most object and merging the shapes together with a compound path (or simple "merge")... you should then be able to mask/divide any objects below it, etc.
Groups are not appropriate for dividing/cropping as illustrator doesn't know how to treat the overlapping shapes (in which order) when adding a new shape to the mix.
If you explain what you mean by "united" shape, I can probably explain it a bit more?