To sort of just recap what others have said

• Option One: Driving all the way in - you obviously need a car for this, it's likely (not guaranteed though if traffic is gnarly) your most direct/fastest option but also possibly the most frustrating. You also have the most immediate access to your car which is nice if you need to leave early or late. For parking, I agree with others that if you're parking in the city, find a garage or lot on something like SpotHero and have it ready - it's not insane to park in the city most of the time but it's likely worth the money to just pay to do it. The biggest frustration might be getting to the parking
• Option Two: Driving into a T Station of some sort - it likely depends on where your meeting is as to what station but either Alewife (Red Line) or Sullivan/Assembly/Wellington/Oak Grove (Orange Line) are usually available and you can use the T to connect to nearly anywhere else you have to with a short-moderate walk. The upside of this is you cut out the worst driving (inside and just outside of Boston) and parking/T costs will likely be cheaper than the gas/parking of going into Boston. The downside is you need to budget public transportation and walking time and, if your meeting place is out of the way, it may be a substantial amount of time. Additionally, if you are staying later into the night, the train frequency slows down a lot and stops earlier than other cities do, so you may be stuck getting an expensive Uber to get back to your car - your car also needs a T ride to get back regardless.
• Option 2.5: Driving to a Commuter Rail Station - same concept as the above really and you can get in from slightly further out, but you're taking an additional train on weird schedules.
• Option Three: Taking a bus - likely the Boston Express or Concord Coach - to South Station and use the T from there. Bus tickets vary from location but if you're picking it up in NH (likely in Derry/Londonderry for instance) it'd be $20ish each way and at set times. The upside is that you're doing no driving in Mass and it's really no slower than driving into Boston. The downside is you are really subject to the bus schedule - as it goes into the evening they become very infrequent with a potential two hour gap near the end - and if you miss that you are stuck except for a very expensive Uber ride all the way back to NH (eek).

Really, all of this is very dependent on the nature of your meeting, flexibility requirement for time, and overall tolerance. There's no perfect answer - my personal preference for going into Boston is #2 but any of those are defensible depending on the situation. Plan it out for sure.