You've definitely found a bug in Cloud that was recently introduced - sorry about that - we are working on fixing it ASAP. As a workaround until the fix is deployed, you can choose "New Space" from the navigation panel on the left side of the window - click the Request Upgrade button and then fill out the form that appears - just add a note in the form that you actually want to add members to your space. We'll get all the info we need to upgrade your account so that you can add additional members.
Sorry for the bug - but thanks for letting us know.