follow this article:
https://support.ringcentral.com/article-v2/Setting-up-Group-Call-Pickup-group.html?brand=RC_US&product=RingCentral_MVP&language=en_US

- On the pop up where it says Group Call Pickup Phone Key select [Yes]
- After you finish the section of the article Setting audible/visual call notifications do the following:

Go to Presence and select Permissions in the top left corner.
At the bottom you will see Select users permitted to answer my calls: and then there is a table with all of the user extensions.
Make sure the user extension you've selected gives permission, to whoever needs to pick up their calls. Not sure why this is not included in that RC article. I strongly believe it is a necessary step.
It is also described with screenshots in here:
https://www.etsu.edu/its/documents/how_to_setup_call_pickup_for_ringcentral.pdf