Ok, this will be available in next build release which will be out either end of day today or tomorrow. The below have all been implemented and tested for both default filter and stream filters.
groupjive.my
groupjive.joined
groupjive.owned
my = joined or owned
joined = joined, but do not own
owned = explicitly own
To limit the activity only to group activity you'd used the below.
groupjive.myonly
groupjive.joinedonly
groupjive.ownedonly
Again, this isn't available yet, but will be with next release.