Being signed in to SyncNow is not enough: only a session open on this project counts. The Overview no longer says who is working: it tells you what changed since your last visit.
In Projects, a green dot next to the project's name says someone else is working on it now; pointing at it says Someone is working on this project, and the project's Conversations say who. In Managers, which only the owner sees, each manager's avatar is green while their SyncNow app is online (it was heard from in the last 150 seconds) and grey when the app is closed or the PC is off; pointing at it says Online now or, for example, Offline · last seen yesterday 19:20, and a pending invitation has a grey avatar with Invitation pending. Each row also says when the manager last worked on the project, for example Last active 28 Jul, or No recorded project activity.
To get that green dot, keep the project's notifications on: in Project settings, under Notifications, Show notifications is on by default; Show notifications applies to everyone who manages the project, so if someone unticks it the dot goes off for everyone. The dot goes away by itself when nobody else is working on the project any more, and your own sessions never light it.