When you join Flickr you are given your own "page" where you post photos to your "photostream" and place them in associated albums. You can "follow" people and they can follow you, and you see one another's images as they are posted. The "groups" are pages/galleries where you can additionally post your photos so that you might gain a larger audience. Each group has a curator/administrator to ensure, theoretically, that photos inappropriate to the group can be deleted. There are "discussions" within the groups, but in my experience these don't count for much. Nature photographers might join various bird and wildlife-related groups. There are a LOT of bird groups. Some people try to post their photos to every single relevant group. Theoretically, the more groups you post your photos to the larger audience you have, if that is what you want.
I really don't see any disadvantages to joining groups. Once you join a group you have no particular obligations to the group.