There should be no apostrophe because what you mean is that the decision belongs to the group.

VOA: special.2010.02.11

People often add an apostrophe to "its" in a sentence like "The group made its decision."

VOA: special.2010.02.11

"It's" with an apostrophe is short for "it is," as in "It's raining."

VOA: special.2010.02.11