Add a getUsersFromGroup helper function

When using a multiple tasks node, if you decide to assign it to a group, as soon as a group member completes the task the task is finished.
We need a function that will return each member of the group so that the task can be assigned individually to them. Then the task will not be finished until every member has completed the task.

=> Add a getUsernamesFromGroup function to be able to assign the task to each group member
=> It should include an option to resolve sub-groups (or not)