The create function for groups in Mendeley Web is currently disabled as the developers are working on updating it.
NOTE: You will only be able to join Public groups. Institutional groups are limited to members of a specific institution (e.g. school). Invite-Only groups will appear in the search but not display a Join button; to join an invite-only group, you will have to contact the group owner. Private groups will not appear in the search at all.
NOTE: Only group owners have the power to delete a group.
You only have so much space available in your Mendeley account. In order to get more space, you will need to upgrade your subscription. You can check the amount of free space you have by going to the "Settings & privacy" option in your profile icon’s drop-down menu and then clicking on the Subscription tab in the left sidebar.