by moodle question.
hi
thanks for replay
yes I logged in as student B and all activities for other groups was available to me (student B)
I know the answer now
dividing students to group and grouping is not enough, we must enable "available for group members only" feature.
as an Admin
site administration>development>experimental setting>then enable "available for group members only"
then this feature will be available in show advance in common module