You are a member of an Office 365 Group but this group is not included in the listing of groups on the left side when you open OneDrive app on portal.office.com
To resolve this issue, you need to "follow" this group:
1. Sign in to portal.office.com and click on Outlook icon
2. If you are not seeing the list of groups, click on Folders to move out of folders area. Locate the group's name on the list of all groups that you are a member of on the left side, and click on it.
3. Click on Files on the site menu ribbon.
4. Click on Browse library (top right corner)
5. Click on Not following, which will change to Following
