So I've more or less been playing since day one. I have tons of alts, several of which are tanks. I stopped doing Flash Points after Mandalorian Raiders because I have haven't been to anything after.
Here is my question: How much grief will I get by just saying I don't know the instance and asking the group to bear with me? I kind figure people are just happy to have a group.
Honestly, it will just depend on the people. I was learning most flashpoints recently, and I had some people help me, and some leave the group. At the end of the day, all you need to have explained is how the three boss's work, and usually only the third will actually present a challenge.
Now that I've learned, I am more than happy to help, and will usually ask if anyone needs help at the start of a FP.