This is the same as #15757.
To summarize, the issue is two-fold. First, sometimes a user's last activity timestamp can be saved multiple times into the database. Second, the group member count checks all group member last activity timestamps for the count. However, the count doesn't do a unique check for the user's last activity entries, so for groups that have users with multiple last activity entries, the group member count will be wrong.
Back in #15757, I removed all duplicate user last activity entries from the database, so this should address group member counts for new groups. However, for older groups that haven't added or removed any members such as the DHUM 7000 group, the group member count can still be wrong if users in the group had multiple last activity entries in the past.
For now, I have manually refreshed the group member count for the DHUM 7000 group; I can probably write a script to run through all ~2300 groups to fix the group member count tomorrow.