Resource allocation in groups

I created a group consisting of multiple resources. When I assign the group to tasks, Omniplan allocates multiple resources to each task, producing a very tight plan - artificially tight. My tasks aren’t really well-suited for having more than one resource on each. How can I tell Omniplan to assign only one resource from a group per task?

It looks like Omniplan is adding resources until there is a 100% load on the task. We run our staff at 86% load, so it is (generally) adding 14% of another resource.