What do you mean by "query all groups"? Get a list of group names? I'm going to assume so with my answer here.
Within each item collection for a group do you have an item which holds the metadata about that group? So the other items in the item collection are the users, but the one item is about the group itself? Having the group name and such. I'm guessing yes.
Setup a GSI having a PK of whatever attribute stores the group name. Then with a scan of the GSI you can get all the group metadata items and nothing else (because nothing else is in the GSI), so it's efficient. Project into the GSI whatever attributes are needed for that "query all groups" query.
Again, I'm making a lot of assumptions here on what you really intend.
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