You can invite people from your workspace team to join a subject:
while creating the subject
at any time by visiting the subject page
You can access a subject's page:
directly from the pinned and suggested subjects in the left column,
by searching the subject name from the top search bar
via the SUBJECTS menu in the left column
by clicking on a subject's name from a task of a conversation,
etc.
Then use the 'invite' menu at the top and select 'Add Members':
You can then choose from the users who are already in the workspace team:
If you need someone who is not yet on your workspace team to join a subject, there are 2 options. You can either:
add that person to the workspace team (and then to any subjects), or
invite that person as a guest on that specific subject only.