Can we limit who can add/remove people from a chat?
No. Every member of a chat can add or remove members. This is like texting. If you would like more control over the membership of a chat group, consider using a Google Space.
Can we add outside email addresses to a Google Chat group?
No. Generally, these “outside” numbers belong to adults, so you are free to text them as you normally would. These adults should be kept separate from communications with students.