EndNote places your newly added references into the "unfiled" group/folder automatically. It is always a good idea to organize your new references right after you added them to your account.
Check the references you would like to add to a group, and select the group name from the drop-down menu or create a new group. You will be prompted to name the new group.
Every reference can be added to multiple groups. To see which group(s) a specific reference belongs to, click on the folder icon under that reference.
You can manage and share your groups under "organize" --> "manage my groups." Then enter the email(s) of people who you would like to share the group with. They also must have EndNote accounts in order to see the shared group. You as the owner of the reference group can decide the level of access they have: read only or "read & write" (they can add, remove or edit references in the shared group).