This a tricky one. > If the group is at least somewhat big, chances are they'd have been approached by companies before The approaching took place in the past

This a tricky one. > If the group is at least somewhat big, chances are they'd have been approached by companies before The approaching took place in the past, so I'm not sure I follow why using "is" makes anything present tense other than the conditional clause? The group must be "at least somewhat big" in the present for the conditional clause to be true, and the main clause is somewhat separate. The expansion of "they'll have been" is "they will have been", which sounds perfectly fine. In my mind the expansion of the original sentence would be, "If the group is at least somewhat big, chances are they would have had been approached by companies before". In spoken communication you would likely drop that "had", but it's also perfectly normal to contract it as OP did. But in written form I think it's too much of a stretch to replace "would have ha" with an apostrophe.