Performance monitoring in BPM server

To monitor active session count and transactions,following counters from Performance modules:

Transaction Manager
Servlet Session Manager
IBM_BPM_Portal_BPM75Node01_server1#portal.war  under Servlet Session Manager.

Limit on number of users in Participant Group ?

There is no limitation on the number of users in PGs, it is recommended to keep the number as low as possible and only tests in your environment can give an answer on the number of users in PG that your system can bare.

Read and Write documents in BPM

Taking into account that one need to have authenticated file access - 

- use some FTP or WEB server to store all files and configure required security level for the file access. This will require an additional implementation for the file upload in case the FTP\WEB server and the BPM server are on the different machines.

Segregate process applications Inboxes within the Portal

Manage the personalized, shared Inboxes that are exposed to each user group.  For example: create Shared Inboxes for all group A users but do not want the Group B users to have visibility to those specific Inboxes.

The exact feature does not exist as an out of the box IBPM 7.5.1 Portal feature.