Problem:

I would place Views on main page of BuddyPress group to display posts of users who belong to this group.

There is possibility in BuddyPress page to place Views but how to select posts respective users of the group?

Solution:

Buddypress plugin stores "user group" in a custom database table "wp_bp_groups_groupmeta", which is not a wordpress user meta or post meta, and it is out the range of WordPress, so you can not filter the result by Buddypress "user group" in Toolset plugin

You might consider custom codes, I have searched in google, and find some related results, for example:
https://buddypress.org/support/topic/filter-users-by-groups/

Relevant Documentation: