Specific to sub-spaces, only those with "manage community" admin rights can create or edit spaces/sub-spaces. Since you started this trial instance, you are the only person able to do this currently.
To answer the other part of your question though, you can share this permissioning however you'd like. Simply navigate to the admin console and click on "permissions". Once there you can add any additional users to be admins with you.
Hope this helps.