This is a PRO feature – you will not see it unless you're using the PRO or tournament version.
Under Officials/Referees (on the Schedule Maker page), click Advanced Options
On the resulting modal, enter the score keeper's name and email, and select score keeper from the dropdown:
You can only add users that have an existing account on the site.
Either a) send them the link to set up an account: https://leaguelobster.com/accounts/register
or b) create accounts via the users dashboard (see help article)
Note:
Score keepers will not be listed in the Officials list seen in the first screen shot, but only in the advanced options dialog.
To remove a score keeper / revoke score keeper permissions, simply clear both the name and email fields and click save.