Hi - at our company we use Groups for self-organizing and self-grouping. We let anyone in the system start a group and it's their responsibility as the moderator to invite others and gain interest in the group. By contrast, Spaces can only be created by administrators of the overall system and there must be a clear goal and business need for the space. We also have defined specific use cases for spaces and we do not do that for Groups - Groups are organic.
That's what we do, not sure about others or a specific glossary.