Here I mean by Investor a person who starts a bounty for somebody else's question.
How can I discuss with an investor? does he receive comments?
You can notify a user who set a bounty on the question by leaving a comment with @TheirName under the question. Although the name does not autocomplete, the ping reaches the user. This works even if the bounty has long expired.
And then I found an older post confirming this. It seems that everyone listed in the revision history of a post can be pinged, except for close/reopen voters without a binding vote (i.e., non-mods and non-gold-badge-holders).