For anyone not familiar with the T visit
www.mbta.com
It's the website for the T. They have trip planners on the website too, so you can plan out your commutes, roughly... The T in general is pretty good though. It at the minimum will tell you an approximate time to go across town. This is not perfect though, there are backups occasionally and other things that will cause delays, especially around rush hour.
Also Green line trains can become very packed, around rush hour/busy night times, so make sure you can split up your group just in case you have to. I think in general though you are far enough out going inbound where it shouldn't be too bad.
If you are taking the T to the Museum you'll need to transfer to a Lechmere train. Lechmere trains do tend to be E trains but they are only specific B, C, D or E Trains when they are headed out of the city towards the end of the B, C, D or E line. When they are coming inbound they are either Government Center, North Station or Lechmere.
Just some experiences from taking the T for 4 years. Hopefully this will help some of you.