Could you please descibe that feature request in detail as a new task on
bugs.flyspray.org?
How are your 'clients' are currently organized?
Is a client = 1 flyspray user account?
When and how should be a 'client' 'attached' to a task? When a client commented to a task? When a client is subscribed for notification on a task?
Filtering by a client/user is currently possible if the client is the task creator.
How many 'clients' can be attached to a task (1 or n clients?) Do you have all clients in a 'client group' (permissions)?