its a bug in 3.1, hiding the party popup right after doing an AcceptGroup() causes you to decline the group invite. For now here is my temp fix until Jafula comes back, edit the file Jamba\Modules\JambaGroup.lua, go down to around line 530 and comment out the hidePopup = true; line. it will look like this:

-- Accept the group invite if allowed.
if acceptInvite then
AcceptGroup()
--hidePopup = true;
else
-- Otherwise decline the invite if permitted.
if autoDecline then
DeclineGroup()
hidePopup = true;
end
end

just add -- to the line after AcceptGroup() ==> --hidePopup = true;

You will still have the ugly party invite group on your screen, you can do the regular macro to accept-all to get rid of it or it'll go away after a few mins. Unless Blizzard fixes this bug the only real fix is to fire an event like 3 secs later to hide the window, instead of doing it a few lines down.