Lulu, that will be an excellent question for the various trip insurance companies you might consider.
I "think" (not positive) the answer will depend on if all 5 people are considered (the term I just made up) a "traveling unit." Will you be purchasing travel insurance for them as a group? Or will some of them be purchasing their own travel insurance and their own airline tickets, etc.? And, it might depend on why one or more of them cannot take part in the dinner cruise....is it a covered reason? Let us know what you find out.
Often (we are most familiar with TravelGuard and GeoBlue), trip insurance companies will allow claims for an insured if that person's travel companion has an insurable issue. Not sure if that is the case if they are insured with a different company or different policy ....I just do not know. But, the "devil is in the details," and none of us on the forum is truly qualified to answer that without knowing every detail of your group's travel....AND the particular insurance company you will be insuring thru....and ditto for your others in the party of 5.
You might just give TravelGuard a call just to see how that company would answer the question, based on the particulars of your group of 5.
Oh, and I looked on their web site: There is a "strike list" (for risks of strikes that they will not cover claims if due to that event) and an "alert list" (which lists the companies for which they will no longer provide coverage..typically for $ instability reasons) if you try to insure AFTER the date on which they appear on the "alert list"....ditto for strikes. What is interesting is that the strike list goes back a few years, so it might be worth confirming (if you plan to fly with one of the providers or into that area) is the exclusion is still in place. Seems they used to list when the strike exclusion no longer applied....but it seems to be left for perpetuity now (maybe...or maybe they still see risk with those providers...do not know).
I will say that we have had two claims with TravelGuard over the years, and they were absolutely excellent, not only with reimbursements, but helping us to fly home (and cover all expenses down to the taxi from the airport to our home upon return). But, that said, our claims were "buttoned up," and I had documentation (including a letter I politely requested and received from a ship captain (when the brand's cruise director could provide nothing)...when a cruise was halted mid-way in process)....TravelGuard jumped into action and got us on the next flight out of Halifax, where other passengers endured a 10-12 hour Hellish bus trip (we were told with only one stop) back to Portland. That cruise line ultimately went bankrupt, and I actually called TravelGuard to warn them to put it on their alert list (but it took them a while to get it there....do not know why the delay...maybe they had to be careful to confirm...do not know). From that point forward, we decided to ONLY stick with well-established brands.
Another claim, the head of customer service at the time, told me that they held up our claim form and documentation as the gold standard (for their customer service team). We probably shocked them with the organization and detail. I am sure they receive a lot of sloppy ones with minimal records, which results in complaints,no doubt.
Understanding a policy and being a good customer is not a bad thing. A lot of front-end time, but it can save a lot of heart ache and frustration later.
End of my yammering.............. Have a fun trip, and I hope all goes smoothly and without a hitch.