Please note that Pin functionality is only available on the Forum Discussion. There is no option to pin a post on the Group Activity. 

You can try to use a third-party plugin called BuddyPress Sticky Post. As its name suggests, BuddyPress Sticky Post lets admin pin up site-wide and groups activities to the top of the BuddyPress activity list. Admin pin any number of activities to the top of a activity and groups page, helping him surface important activities at a glance. Click on the link below for more details: 

https://wbcomdesigns.com/downloads/buddypress-sticky-post/

As this is a third-party plugin, we are not able to guarantee support and compatibility as we have not fully tested it.