[Possible Bug Solution] Your party bugging up in Cyrodiil? This might be why.

Baphomet

We have found that one of the reason why people sometimes are showing as offline, can't be invited into the party and/or crash in Cyrodiil is if they get added to the party while their character is outside of Cyrodiil. For some reason that seems to cause a lot of havoc to the group as a whole.

If you are having issues with your party, try to dispand it and reinvite everybody while their characters are in Cyrodiil. Hopefully that will work out for you as well as it has for us.

Game on!