In Managers, a Pending invitations section, with how many there are, appears while invitations are waiting: each shows the address, when it was sent and when it expires. Once accepted, they appear among the managers.
Whoever is invited can accept or decline from the invitation in their project list, a row like the projects, or accept from the link in the email; if their SyncNow is open, a notification tells them that a new invitation arrived. After Accept, SyncNow opens the project on its Overview as soon as SyncNow's server confirms the new manager, usually within a few seconds; if it cannot confirm it within about 20 seconds, the app stays on Projects, the status bar says why and the project is in the list to open a moment later. A declined or expired invitation simply leaves the list.
To withdraw a pending invitation, click Revoke invite next to it and confirm: the link in the email stops working. Sending a new invitation to the same address replaces the pending one, and it is also how you invite someone again after an invitation expired or was declined.