When you @ mention a colleague or assign an email to someone who isn't part of the Shared Triage, they'll gain access to the conversation and see the email in their personal Triage. They will then receive updates on further activity. However, they won't have the capability to reply directly from the email account.

Similarly, when you add an email to a timeline, it becomes visible to any colleague with access to that timeline. They can reply to the email from their personal Triage but won't have the option to reply using the email account directly.