Can a group health plan require participation in a wellness program or a wellness assessment as a condition to participation in the plan?

No.

why not? HIPAA's nondiscrimination rules?

Sorry my first response was so brief--I was interrupted. I am basing this on the clarification of HIPAA nondiscrimination rules that was published earlier this year. The clarification says "participation" in a plan can not be denied because of pre-ex, risk-taking activity, etc. It does say you can pre-ex a limited time and exclude claims for injuries resulting from risk-taking activities, you just can't deny participation in the plan in the first place. I think the only way you can utilize the wellness program factor is in your ability to set premium differential based on participation in a wellness program. Having said that, if the person can demonstrate that they are unable to comply with the wellness program (smoking cessation for example) because of an underlying medical conditon (nicotine addiction--inability to quit) then you can't use the differential against them.