Display child records in parent table.

Is the list you want specific to the parent? ie, do you want to list all the children in a dropdown? Or is it that you want a list of any shareholders, based off a generic filter?

If the dropdown needs to be conditional based off the parent, the only way I can think to do this (if you don't just want to show them on a report link), is to make a reverse relationship, and have Shareholders be a parent to Startups. Then use the conditional rules.

If option two, you could go to your shareholders table and make a formula text field called [Shareholder Names for Multiple Choice List]. In that formula, essentially only show the names of the shareholders you want based off whatever If statements you need. Then reference that field from your Multiple Choice.