Group reviewing a manuscript sent our for review. Ethical?

Grey area, leaning towards unethical. Confidentiality is an important value in the peer review process, and the usual understanding of confidentiality is that the paper will be distributed to the minimum number of people required to perform the reviews. Therefore, your case, where a group of people replaces a single person, can be seen as a violation of confidentiality.

Still not enough detail for a definitive answer, but here goes:

Confidentiality is fundamental for the review process and especially the authors who submit their papers expect that their manuscript is treated confidential. Hence, all reviewers have to treat the manuscript as such (and usually the editors informs the authors of the general review guidelines). If you as a reviewer asks the editor if other may help you with the review, this is totally OK. The editor may decide what to do, especially he may decide to allow this procedure but all members of the group performing the review have to adhere to the review guidelines, especially treating the whole review process confidential. Also the editor should know all members of the group who is performing the review. If all this is the case, there is nothing wrong with a group review.

Of course, you may not extend the group of reviewer at will.