You can set the rule as follows:
"Node": {
"$field": {
".read": root.child('Projects/' + data.child('project').val() + '/Members/' + auth.uid).exists()",
".write": //same here
}
}
"Node": {
"$field": {
".read": root.child('Projects/' + data.child('project').val() + '/Members/' + auth.uid).exists()",
".write": //same here
}
}