The three main types of Qatalog users in an instance are:
What type of users counts toward my seat count?
Both Admins and Members count toward your active seats, Guests do not.
What access do invited guests have?
Guests only have access to the items you have directly invited them to. You can invite guests to a workflow, or a specific work item in one of your Modules. (e.g. Add them to Client A in your Clients Module).
They will be able to view, edit and collaborate on that specific item, but will not have access to anything else in your Qatalog.
How do I invite a new guest?
First, make sure that external sharing is enabled on the Module you're working on. You will need to be an admin to do this, and go to Settings > Modules > Select your Module > General > Allow items to be shared externally.
Now that that's enabled, simply go into one of the items in that Module, click on the three dots in the upper right corner, and select "Share externally".
The same guest can be invited to more than one item.
To learn more about external sharing, you can read this article: External Sharing
Can a guest be added to a team?
No, Guest accounts cannot be added to teams. If you want to invite a user to a team, they have to be members of your Qatalog instance.