Use context files, and send and apply with patch bundles.

A context file is created with

darcs changes --context > con_A

and contains everything darcs needs to know to "fake" a send to a repo. These context files can be distributed over the group.

Person A:

darcs send --context=con_A -o bundle .           ### con_A (and not B) is still person's A context from the previous step
[interactive]
mail B < bundle