It is possible to create broadcast group using non-existent connectors. It seems that validation is missing for add operation. This affects both CLI and Amin console, thus I suppose that problem is in underlying Management API. 

Example:

1. Creation of new group with non-existent connectors "ab" and "ba" will succeed 

[standalone@localhost:9999 /] /subsystem=messaging/hornetq-server=default/broadcast-group=foo:add(socket-binding=default, connectors=[ab,ba]) 
{
    "outcome" => "success",
    "response-headers" => {
        "operation-requires-reload" => true,
        "process-state" => "reload-required"
    }
}

2. However if we try to change the value of connectors attribute to the very same value it will fail
 
/subsystem=messaging/hornetq-server=default/broadcast-group=foo:write-attribute(name=connectors,value=[ab,bc])
{
    "outcome" => "failed",
    "failure-description" => "JBAS011680: The broadcast group 'foo' defines reference to nonexistent connector 'ab'. Available connectors '[netty, in-vm, netty-throughput]'.",
    "rolled-back" => true,
    "response-headers" => {"process-state" => "reload-required"}
}