Workflows: process initiator assigning roles in "set up participants" activity?

Hi,

I have a workflow which starts with a "set up participants" activity and would like the following to
occur:

User1:
1. Initiates process.
2. User1 receives an assignment requesting that individuals be assigned to roles.
3. ... workflow continues ...

User2:
1. Initiates process.
2. User2 receives an assignment requesting that individuals be assigned to roles.
3. ... workflow continues ...

In other words, I would like to have the initiator of a workflow become the assignee of the
"set up participants" activity.

Any answers or pointers to relevant documentation would be appreciated.

Thanks.