One possible solution:
You can use our Group Lookup component to get a comma-separated list of the emails of the members of the group. Then, in the step before the User>assignTask step, send the group an email. The only problem is that you won't be able to embed a link directly to the task, because the task doesn't exist yet.
To work around this problem, you can use the technique outlined here:
If you need more assistance, contact info-at-avoka.com