Hi Richard,
You can use this guide to create a language string override.

You can find the language constants for the EasyDiscuss Group app here:
JoomlaFolder\administrator\language\en-GB\en-GB.plg_app_group_easydiscuss.ini
This is the language constant for Easydiscuss app name in the sidebar: APP_EASYDISCUSS_GROUP_TITLE

Is the EasyDiscuss Group App (cost $25) something completely different and requiring a payment no matter what subscription I have?

The EasyDiscuss Group App is different from the free EasyDiscuss(v1.3.7) app. The free app does not have the EasySocial group integration with EasyDiscuss, only the ED group app does. The free app also cannot be used to create group discussions in ES group pages. The free app is more for individual user uses, for example creating a discussion from the story form in the public dashboard.
The app does require purchase regardless of what subscription you have on any products. The cost for the ED group app is a one time payment, no recurring or subscription fees, you can also update the app without additional costs.

Thank you for the information Raymond, it was very helpful, override now in place for the Title and $25 Group App purchased/installed.

Hey Richard,
You are most welcome and thanks for your support. Glad to hear that your issue has been resolved now.

As a gentle reminder, kindly start a new thread if you have any other issue in the future so it will be easier for us to manage your inquiry. I will lock and mark this thread as resolved.