Just an idea, maybe you can use sharepoint  Object Model to find out
the members of the sharepoint group, then add each member's email  to
the To property.

Yeah, that's the route I'm taking for now. I'm find
the SharePoint object model to be more complicated
than I anticipated.

The WorkflowContext has a property  named Web that references the web
containing the list that is associated with the workflow. Using the
SPWeb object, you can get group  members. Below is sample code to list
members of a group. HTH.

string url = args[0];

string groupName = args[1];

using (SPSite site = new SPSite(url))

{

using (SPWeb web = site.OpenWeb())

{

SPGroup group = web.Groups[groupName];

Console.WriteLine(String.Format("Users in group '{0}' in site
'{1}'", group.Name, url));

foreach (SPUser user in group.Users)

{

Console.WriteLine(String.Format("{0}\t{1}", user.Name,
user.Email));

}

}

}