Sometimes new employees for a company do not want to enroll immediately for the group insurance coverage. They must realize that if they do not enroll at the end of the stated waiting period for new employees (the waiting period can be 0, 30. 60, or 90 days and is determined by the employer at the time of signing the insurance contract) then there will usually be a 12 or 18 month waiting period before pre-existing conditions are covered.
Sometimes workers returning to the USA on furlough drop their group insurance while on furlough and then wish to be re-instated once returning overseas. This may be done but again the insured must realize that there will be an 18 month waiting period before pre-existing conditions will be covered.