If we add external guests to a RC team chat, can they reach out to / start conversation with rest of our employees?

​@Nishant Gauba,
 

Yes, external guests can initiate direct messages with other users in the RingCentral App—either from their end or vice versa—unless restrictions are applied.

As a workaround, an admin with Administration Access in the RingCentral App can manage this by adjusting the external guest settings:
RingCentral App > Settings > Administration > Messaging > External Guest Settings

Once the setting is restricted, any guest users who were already part of a group message will no longer be able to initiate or send direct messages to users within your company.

If the admin needs to add new guest users in the future, they can temporarily lift the restriction, add the necessary guests, and then reapply the restriction once the users are in place.