HI @Seth_Giammanco 
and thank you for reaching out!
As it stands, there is no way to restrict people in a @list to only be available within a particular Team or Project.
We already have a thread regarding this topic int he #productfeedback category. If you haven’t yet, please feel free to add your vote and comment to it.: Restrict Guest permissions
If you have any follow-up question or there is anything else I can help you with, don’t hesitate to @mention me, I’ll be more than happy to help! Have a great Tuesday!