In the meantime, my solution: Identical observation set + my browser extension
I started using the Similar observation set observation field some time ago to link observations of multiple users that depict the same organism at the same time. It works by choosing one observation of the bunch and inputting its id (e.g. 356586999) as Similar observation set under all the observations that should be linked (one could also choose some other identifier, but it must be unique, which is guaranteed that way).
As an example: https://www.inaturalist.org/observations/356586999
One can see all linked observations, by clicking on the observation field and then clicking → Observations with this field and value. To make it more user friendly, I coded an extension that displays linked observations under each individual observation as a grid.
This also works with other fields like Identical observation field, which work the same way, but should only be used to link observations of the same individual.
The Similar observation set field on the other hand is also used sometimes to link observations of different organisms found in the same spot or similar use cases.
(To group observations of the same species found together, the Group observation set field would be most suitable.)