Task Assignment using variable

[natacam]

Hi
I´m trying to assign a task using the assignment expression:

<task name="Iniciar proceso de planeación">
 <assignment expression='variable(groupName)-->member(Gerente de Proyecto)' />
</task>

In the first term, i want to get the Group with the name "groupName", but it seems that it tries to get a user with name= groupName.

Can`t I get a specific group with the expression "variable(...)"? how?
Thanks for your help