Right now, any user can initiate a private chat with anyone in our entire organization. Suggested contacts list people in completely different departments even though they've never had any interaction of any kind with them. We have users in a child company that has nothing to do with the parent company and they show up in each others suggested contacts with no way to segment them. We have mailbox users for logging and automation purposes and are configured to not be displayed in the GAL, yet they show up as suggested contacts even though they are not licensed for teams!
Is there any way to limit users to only have the ability to search and initiate private chats with members of their respective team and/or group and to restrict who shows up in the searches or in suggested contacts?
Guests should only see team members, outside of that segmentation is inadequate but there is supposedly something in the works. That said you could disable private chat entirely for select users using messaging policies in the new Teams & Skype admin center.
You are correct, the "guest" user I was referring to was actually an external user that was provided a limited/temporary Office 365 F1 account. Sorry for the confusion. I edited the original question to not confuse others. Disabling Private Chat is not a viable option as they need to both participate in team discussions and be able to privately chat with other team members.