Regarding your queries :
>>>> Is it possible for customer (a) to create a report and to then share that report with customer(b)?
Yes, it is. You need to create your own custom security provider to implement this.
Please refer to the following documentation links which discuss the same :
>>>> do you have steps or a link to a guide on how I can share the reports, i’m still quite new to Active Reports Server.
You need to impose the required permissions on your resources(reports, models, schedules, etc) so that they could be shared with other users. I would request you to please refer to the following documentation links for more information on these :
Hope it clarifies.