Users only require a single user login for the whole of Elements. Access is governed at the Space level.
To access any content, the user must first be invited to or provisioned into a Space. If the email address used has not been registered before, the user will first need to respond to the verification email before they can be assigned permissions, (e.g. Edit, Manager on content) - but they can have content shared with them.
The Space Admin(s) set permissions (Edit/Viewer) on the space, based on the number of un allocated editors on the space. As long as editors remain, they can assign edit rights to any number of users on the space.
Once you are a verified Elements User and member of a Space, other users can share content with you. If you have been granted Edit rights, they can also opt to provide you with edit rights to their content. By default you are a viewer when someone shares content with you. Edit rights must be explicitly granted. Once you are an editor, you can also create content yourself and/or copy content into this space from other spaces.
This makes Elements a very secure and well governed repository of knowledge, while allowing those with edit rights to create/copy in content in private, only making it "visible to all space members" when appropriate.
You can also keep the "Visible to all space members" toggle off on content, and invite specific people with view or edit rights.
If you have content which you do not want any use to be able to copy, you can switch the "allow any user with access" toggle off.
Inviting to a space by simply inviting to specific content
You can shortcut all of the above by simply sharing content from the space with other users. Any user who gets invited to or has content shared with them from the space becomes a member of that space.
You cannot, however, invite other users to your Personal Space. As a result, you cannot share content from your Personal Space, as nobody can have access to its content without being its member.
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 a PRO or Enterprise space, 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 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 affects 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).