Cannot "Share with group"

Our group structure looks like this:

I am a member of SRE, and own a repo in the SRE group. I'm trying to add the "volum" group to the repo, using the "Share with Group" function. However, I cannot see the "volum" in the dropdown, only the subgroups of "SRE".

The documentation doesn't mention anything about Group "visibility". I can see the Volum group when I browse "all groups" in the "Groups" link in the top navbar. How can I make sure groups can share with each other?