Is there already a way to pin certain posts? Would that be a solution?
Also had that thought. 
I think it would not solve it entirely, since pinning is usually for the whole group, and some people have their specific messages they’re interested in. But still it’s something nice to have!
Another idea is to mark messages as favorite, with a little star somewhere.
From Daniel, Luxembourg:
On place pages, the description is above the wall, so important details can be added there. On the general wall of a group, there is no such possibility. Therefore, adding the option to pin one or multiple posts could be a flexible solution. This could be used to share important announcements or to welcome everyone.
I suggest to limit the number of pinned posts as people should not have to scroll over too many posts to read the wall’s newest posts neither.