It would be nice if there could be a permission level for "in the same group(s)". Then issues could be automatically available to users in the same group(s).
It could either compare all groups or then some part of the group names, for example starting with "customer-".
I would similar to the "Reporter" visibility setting, for example "Reporters group" and then you input the prefix of the groups to search from.
This feature would make it easy to setup customer groups, where all customer reporters could see their companys issues. Then you would not need to setup different security levels or projects for all customers.