Hi,
does your Schedule-Tab look a bit like that?
Probably all with the No Information-Graphic 

If so, i guess you or your Colleagues haven't set any Permissions in their Calendar and not to forget you also need to at least add Foldervisible to your Mailbox-Root
This can easily be set either via Outlook or Grommunio-Web with a "Right-Click-> Share Folder" by the respective User or someone with Full-Permissions on the above mentioned Folders.
You could also do a little Shell-Magic and set these Permissions via grommunio-admin exmdb mbx-usr@mail.org folder...
So just as a quick example, log in to your grommunio-host via ssh and execute the following Commands
# This Variable will be used to identify the Mailbox on which we want to set some ACLs
MBXOWNER="user@domain.org"
# This Variable will be the User who will be granted some permissions
MBXREADER="otheruser@domain.org"
# Now we will set the minimum ACL on the Store
# 0x9 is the static ID of your Top of Information Store
# check via `grommunio-admin exmdb someuser@domain.org folder list`
grommunio-admin exmdb "${MBXOWNER}" folder grant 0x9 "${MBXREADER}" foldervisible
# And now we will set some ACLs on the Calendar (0xf)
grommunio-admin exmdb "${MBXOWNER}" folder grant 0xf "${MBXREADER}" foldervisible readany
More on the possible ACLs
@jengelh on this topic, can you provide an ETA for Standard or even Anonymous-ACL's?