If you're trying to decide between inviting a user as a member or as a guest to your workspace, the key thing to know is that members have more options and control over their Height experience than guests do.
Guests are able to collaborate on tasks within lists that they have access to. This means that they can make comments, mark tasks as done, and create new tasks within a list they have access to.
Some examples of what guests can and cannot do:
Guests can only see and discover lists that they have been added to by email / name.
Guests cannot change View menu options for a list they have access to.
Guests can use Filters on lists they have access to, but cannot save those filters as a new smart list.
Guests can be added to smart lists, but they'll only be able to see tasks from lists that have been shared with them.
Guests only know about (i.e. can see the name / avatar in places like "Assignees") of other users who have been explicitly added to the list they're on.
Within Settings, guests can only see and make changes to Personal settings like Notifications, Profile, and Preferences.
Guests cannot use the Share menu to add any other users to lists.
Guests cannot invite new users to join the workspace.