Support
The ninjas app is for the members of the ninjas team: they keep the team's projects and posts up to date from their phones. Only Google accounts the team has invited can sign in; there is no sign-up.
I can't sign in
"Not a member" means the Google account you chose isn't on the team. Sign in with the account you were invited with, or ask a team owner to invite this one.
I don't get notifications
In the app: Profile → Settings → Notifications. If it can't be turned on, notifications for ninjas are off in your phone's settings.
Anything else
Write to moustafa.mesbah@gmail.com. We usually answer within a few days.