Currently there is no feature to only share a custom report with one other user. When selecting to share with All, this report will be available to run for all users with applicable access when accessing Custom Reports from the Reports tab.
That being said, this is applicable access is for users who were in the system when the custom report was created. Any shared reports will not populate of a user added after the report is shared.
To allow new users to access the report, you will need to Save and Share the report again after the user has been added, this way they will be included. Alternatively you can always send these reports to users instead with the below steps.
1. Go to Reports, then select Custom Reports
2.To the right of the group name, select Edit.
3.Turn on the email schedule and set up as required. Enter the email address of the user, then select Save and Close.