First, verify the member is in the group assigned to that discussion
Here's how -
Click on the Members tab/search for the member / click on the member, you can view all groups they are currently part of at the bottom of their profile.
*Approving a member does NOT assign them to a group, that is a second step.
If you need to add a member to a group -
Here's how -
Click on Groups / Select the group / Click on Add Members / Search for the member and check the box / Click the orange button at the bottom "Add Members" ... that's it!
* Only members who are NOT currently in the group will be displayed.