There are some topics on the forums regarding this subject.
Including:
https://forums.elderscrollsonline.com/en-GB/discussion/295333/getting-kicked-out-of-group-for-no-reason-in-cyrodiil/
There is the GAME-BREAKING issue with grouping in Cyrodiil at the EU side (can't confirm the US side) of the servers.
On the EU servers the pvp-servers are slowly bleeding to death, because grouping is impossible due to the bug.
In short, this bug kicks you out of group each time you respawn at a keep, camp or outpost and each time you use a transitus shrine.
Which is hitting PCP communities and guilds hard. Simply because it is impossible to play as a group.
To my unpleasant suprise the fix (which is available according to ZOS_BrianWheeler, see previous forum post), is not in the patch of today (based on patchnotes).
When is this "fix" going to be implemented? Because this really is game-breaking.
"Update 12 brings players together in One Tamriel" (ZOS, 2016), unfortunately it is the opposite that is true for PVP grouping.
Only responds to constructive replies/mentions
Immortal-Legends Guild Master
Veteran PvP player