Generally speaking it's not possible to add or replace travelers on an existing policy, so if you've bought your coverage and still have other traveling companions who need insurance they're going to need to purchase their own policy. In most cases they will still be considered your traveling companions as long as they share travel arrangements with you.
The reasoning behind this is that most policy information (such as eligibility for a waiver of pre-existing conditions, or being considered fit to travel as of purchasing the policy) are based upon the day when the policy was first purchased. Changing travelers after the policy is already in effect can lead to problems if a claim should arise.
This leads to another point. Most policies will have regulations regarding who can be insured under one policy. So if you do purchase a policy insuring traveler who do not meet those requirements, it is usually a much quicker option to cancel the original order and rewrite it rather than trying to remove travelers from a policy. Removing travelers, however, can be a bit situational. Some companies may be willing to take individuals off of a policy altogether, and refund the difference. This will not always be the case though, so it is best to contact our Customer Care center if circumstances require you to remove a traveler from your policy.