Mostly the bounty managers who are using different bounty portals ask for the signup on portal by using email, and those emails are then added in the spreadsheet as user id to reward the user accordingly.
"Sign up on portal" are you referring to google forms or bounty platforms like bountyhive?
If its google forms then we are on the same topic but if it is bounty platforms then its probably indicating another issue. Please confirm?
I am referring to both(i.e google forms and bounty portals like bountyhive), they are exposing the email ids (that are being used in bounty portals) in spreadsheets and also if we fill in the google forms for our applications or reports.