Using a common and unique tag for all of the related observations is one option.
A little more visible option (to the community) might be to use a field like Related Observations. You can see an example of its use here.
There are similar fields for the same purpose, some mentioned in this languishing Feature Request:
https://forum.inaturalist.org/t/link-observations/1367
You can also just paste a list of related observations URLs into the observation descriptions or comments.