Hi Nina,
The pin functionality isn't present in the community however you can do the following to make sure this is easily accessible:
1) Navigate to the pull-down for the "Collaboration" tab in the menu
2) Choose "Groups"
3) Turn on the "My Groups" tab
This will show you any group where you're a member.
Another option is to bookmark the link to this community page.
I hope this helps. Please let me know if you had any questions.
