Maybe all group policies are different, but I've changed jobs 3 times and never had to answer to pre-existing conditions.
As others said above, I had to accept it when it was offered or I would have to answer to pre-existing and other questions later. I've been treated for depression for years and it has never come up w/ new policies. ANd they've always covered my same Dr and my meds.
If you decline insurance coverage at first, and then later ask to enroll, I guess they think that some health issue has popped up and you are needing health coverage for it.