Life sciences publishers that support/encourage collaborative manuscript review?
Most of the European Geosciences Union (EGU)/Copernicus journals do something sort of like this - they've been doing it for some years and it mostly works out well. The journals are fairly reputable in their fields and from what I can see authors seem to like them.

EGU describe them as "two-stage journals with public peer-review and interactive public discussions".

Unhelpfully for you, though, these are all geosciences. (List)

The process is:


article submitted
posted as discussion paper after initial approval by an editor
reviews &amp; responses posted publicly, with third parties able to post uninvited reviews


The system still works on individual not collective reviews, but the fact that they're public potentially allows them to build on issues raised by each other, and I can't see an obvious reason they wouldn't accept a review contributed by a group.