What steps will reproduce the problem?
1. Signed up for standby on raid.
2. was accepted to attend.
3. change status to standby
4. accepted as standby

What is the expected output? What do you see instead?
Errors in Chat messages: It reposts the error every 15 seconds.

[18:06:41] [ERROR] [MCEventLib] Error dispatching event GUILD_ROSTER_UPDATE: ...erface\AddOns\GroupCalendar\CalendarGroupInvites.lua:446 attempt to concatenate field 'mGroupStatus' (a nil value)

Opening group calender resulted in another spam. This error repeated a few times while the window was open and after closing seems to be repopping about every 3 minutes or slightly less.

[18:10:46] [ERROR] [MCSchedulerLib] Error calling task anonymous: ...erface\AddOns\GroupCalendar\CalendarGroupInvites.lua:446 attempt to concatenate field 'mGroupStatus' (a nil value)

What version of the product are you using?
4.4.1

Do you have an error log of what happened?
The 2 above and when opening group calendar bugsack caught:

2009/02/28 18:08:43-1061-x3]: GroupCalendar-Are you sure you want to reset the version list?  No events will be affected, but Group Calendar's memory of what version players are running will be erased.  Players will not show up again until they are seen online.\CalendarGroupInvites.lua:446 attempt to concatenate field 'mGroupStatus' (a nil value)
(tail call): ?:
<in C code>: in function `sort'
GroupCalendar-Are you sure you want to reset the version list?  No events will be affected, but Group Calendar's memory of what version players are running will be erased.  Players will not show up again until they are seen online.\CalendarGroupInvites.lua:385 in function `SortGroup'
GroupCalendar-Are you sure you want to reset the version list?  No events will be affected, but Group Calendar's memory of what version players are running will be erased.  Players will not show up again until they are seen online.\CalendarGroupInvites.lua:800 in function `SelectionChanged'
GroupCalendar-Are you sure you want to reset the version list?  No events will be affected, but Group Calendar's memory of what version players are running will be erased.  Players will not show up again until they are seen online.\CalendarGroupInvites.lua:327 in function `UpdateGroup'
GroupCalendar-Are you sure you want to reset the version list?  No events will be affected, but Group Calendar's memory of what version players are running will be erased.  Players will not show up again until they are seen online.\CalendarGroupInvites.lua:114 in function `EventChanged'
GroupCalendar-Are you sure you want to reset the version list?  No events will be affected, but Group Calendar's memory of what version players are running will be erased.  Players will not show up again until they are seen online.\CalendarAttendanceViewer.lua:264 in function `EventChanged'
GroupCalendar-Are you sure you want to reset the version list?  No events will be affected, but Group Calendar's memory of what version players are running will be erased.  Players will not show up again until they are seen online.\EventViewer.lua:357 in function `SaveAttendance'
GroupCalendar-Are you sure you want to reset the version list?  No events will be affected, but Group Calendar's memory of what version players are running will be erased.  Players will not show up again until they are seen online.\EventViewer.lua:1170 in function `HidePanel'
GroupCalendar-Are you sure you want to reset the version list?  No events will be affected, but Group Calendar's memory of what version players are running will be erased.  Players will not show up again until they are seen online.\EventViewer.lua:1096 in function `ShowPanel'
<string>:"*:OnClick":3: in function <[string "*:OnClick"]:1>

Please provide any additional information below.
That's all I have. Hope it helps keep the issue from popping up again.