The Short Version
Figure out what you want before you start. Answer good posts quickly. Keep the first jam low-stakes, and pay more attention to whether people show up prepared than to how well they play. Most bands don't come from one perfect post. They come from a handful of so-so jams until one of them clicks.
Know What You're Looking For
Before you post or reply to anything, get clear on what you want. Covers or originals? Weekly rehearsals or a jam once a month? Are you trying to gig, or do you just want to play loud on a Tuesday night after work?
None of those are wrong answers. But a cover band that wants to be playing weddings by spring and a guy who wants to noodle on Dead tunes in his basement are not going to be happy together. Better to find that out before anyone hauls an amp up three flights of stairs.
Think about geography too, because Boston traffic will make the decision for you if you don't. Quincy and Somerville look close on a map until you try the drive at 6pm on a Thursday. A lot of people put a radius right in their post ("within 30 minutes of Waltham") and it saves everybody time. If you don't have a car, say which T lines you can get to.
Where Boston Musicians Actually Look
Craigslist
The Craigslist Boston musicians section is still the busiest board around here, for better or worse. It's messy, and half of it is people selling gear or offering lessons in the wrong category, but it's where most of the listings on this site come from. Plenty of local bands have found members there.
Smaller musician sites
Sites like find-a-musician.com get a lot less traffic, but the profiles tend to have more detail, like what gear people have and who they're influenced by. They're worth checking if Craigslist is coming up empty for your instrument.
This site
Honestly, this is why I built Boston Bandmates. I got sick of opening six tabs to look for a drummer. The listings page pulls those boards together so you can search by instrument, genre or town in one place. Check back a couple of times a week, because the good posts get answered fast.
The music schools
Berklee and the Boston Conservatory are right around the Fenway, and NEC is a few blocks away on Huntington Ave. That's a lot of very good players in a small area, and plenty of students are up for side projects. The catch is the school calendar. People go home for the summer, and your new bassist might graduate in May and move to Nashville. Go in knowing that.
Open mics, jam nights and porchfests
Meeting people in person is slower, but you get to hear someone play before you commit to anything. If you hear a drummer you like at a jam night, go talk to them afterward. Worst case, they say they're busy.
Porchfest season is great for this. Somerville has one every spring, and lots of other neighborhoods and towns have started their own. Some of the bands on those porches are pickup groups that came together just for the day, and sometimes they're looking for a reason to keep going.
Everybody Needs a Drummer
Spend a few weeks reading these listings and you'll notice it right away. Bands looking for a drummer far outnumber drummers looking for a band. When I went through the Craigslist posts this October, it was something like three to one. Bass players are in short supply too. Guitar players are not.
So if you play drums or bass, you have some leverage, and you can afford to be picky. If you play guitar, you'll stand out by being easy to work with. Show up with the songs learned, offer to book the rehearsal room, and be flexible about the setlist. It sounds small, but that stuff matters a lot when the band has six other guitarists to choose from.
Replying to a Post
Be quick. A good post with a clear description and a link to some music will get a pile of replies in the first day or two, and the people who wrote back first usually get the first audition.
Keep your message short and specific. Say what you play, roughly how long you've been playing, where you live and when you're free. Include a link to something, even a rough phone video of you playing in your bedroom. "Saw your post, I play bass, hit me up" is the kind of message that gets skipped, because there's no way to tell if you've been playing for two weeks or twenty years.
And read the post. If they're doing 90s alt-rock covers and you write back about your prog metal concept album, that one's on you.
The First Jam
Treat it as an audition that goes both ways. Agree on two or three songs ahead of time so you don't spend the first hour trying to find something everyone knows. If nobody has a space, book a room by the hour and split it. Two hours is plenty for a first meeting.
Don't stress about playing perfectly, because that's not really what anyone is figuring out. The real questions are simpler. Did people show up on time? Did they learn the songs? Was it fun? Skill gaps close with practice. Flakiness pretty much never gets better.
You're meeting strangers from the internet, so do the normal things. Meet somewhere public first, or at a rehearsal studio with other people around. Tell a friend where you're going. And maybe don't bring your nicest guitar to a stranger's basement on the first visit.
When It Doesn't Work Out
Most first jams don't turn into bands. That's normal and it's nobody's fault. If it isn't clicking, say so politely and move on. Don't just go quiet on people.
The Boston scene is smaller than it looks, and you'll run into these people again at a show in Allston two years from now. Maybe they'll be in a band that needs exactly what you play. Being decent about it costs you nothing.
If you're putting up your own post, I wrote up some tips on writing a musician wanted ad that people actually answer. Otherwise, go browse the listings and good luck.