By default, any user who is a member of your Space can invite other users, whether they belong to your organization or not. All content within a Space, like diagrams, is also visible and accessible to all members of the Space by default.
However, if you want to have tighter control over who can access your Space, who can add new users, and which content is visible, you can restrict access and sharing on your Space.
Prerequisites
To apply restrictions on Space invites, you need to be Space admin on the Space
To apply restrictions on accessing maps, you need to be the owner or manager on a map
To apply restrictions on accessing Salesforce Org models, you need to be the owner or manager on the Org model
Restricting space invites
By default, any user in the FREE space can invite another user to it, either explicitly or by sharing content from the space. On paid Spaces, however, Space Admins can restrict space invite rights to Space Admins only.
Click on "Change space" -> "Go to Space list" and then for the selected Space change the settings in the right side-bar.
This also affects content sharing. When Space Admins control space invites, users cannot share content from the space with users who are not already members of that space.
Restricting process invites and diagram sharing
By default, any user who has access to the process can invite another user to it. You can do that either by explicit invitation, or by sharing a diagram from the process. On a PRO/Enterprise space, however, Process managers can restrict process invite rights on the right panel in the process list so that only they and users with explicit permission can invite users to the process and share its contents.
By switching the toggle, only Process Managers and users who were granted explicit "Can grant access" rights will be able to invite new users to the process and share its contents with new users.
This also influences diagram sharing. When this toggle is switched off, and users do not have explicit "Can grant access" rights, they can only share diagrams with users who already have access to this process (as sharing with a new user would invite them to the process).