I am probably over equipped for a jam / party band I am in, aside from the fact we have to rent PA, but I digress...
At a minimum you should have.
#1. Your bass in good working order, well set up, nothing broken, or likely to get broken.
#2. A backup bass you are familiar with in case of catastrophic failure.
#3. A bass amp / cab, or bass combo amp. You need to get signal to the board, or sound to the audience depending, and of course, you need to hear yourself, and it is a LOT easier to monitor yourself with a decent amp and speakers. If you are budget sensitive, and want to simplify your load in / load out, give a good combo amp serious consideration.
#4. A tuner. Even if your amp has a built in tuner, for the most part, the built ins in the amps I have used have been, well let's be nice and just say less than ideal. The cheapest way to stay in tune is with a clip on tuner, but they won't mute your signal while tuning. Use a tuner pedal like the Snark SN-10S, or a tuner in a good multi effects pedal like the Zoom B3. I have both, the B3 suprised me by making the Snark irrelevant.
#5. Cables. You need to connect your bass to your tuner, your tuner to your amp, or your bass to your amp, your amp FX out to your tuner, you tuner out to your amp FX in. You also will likely need a spare. The 2 things I have had fail, and seen fail on others in gigging situations are cables, and strings. Spares always help.
#6. Strings. Yes you brought that backup bass, but if you are a Precision player, and your backup is a Jazz, your tone will be off a bit until you get strings on that Precision.
#7. Gig bags or cases for your basses.
#8. Setlist sheet. Nothing worse than starting in playing a completely different song than the rest of the band, and yes I have done that. Don't do it... Make sure the entire band is on the same page set list wise, and play them in order.
#9. Mic, Mic stand, and cable. If you are going to do any vocals, you are probably going to want your own mic. I honestly own cheap azz mics, but they work well enough for me although several members here tell me they aren't usable in gigging situations, experience proves them wrong.
#10. Instrument straps.
#11. Skill as mentioned above.
Assuming you are equipping the band with common gear, and not just the bass players stuff you will need to add.
#1. Mixer with enough XLR, and TRS ports for your band. I prefer unpowered, powered mixers pose a LOT of reliability problems from what I have seen in use with musicians, and sound engineers. My mixer is a cheap Behringer XENYX 1202FX, my guitarist has the same mixer, and we don't mic up the drummer, it is barely adequate for our use. At some point we need to upgrade to at least a XENYX QX2442USB, perhaps with money actually coming IN we might consider buying something a bit more substantial like a Yamaha 24 input mixer, or even a digital mixer.
#2. Stage snake with enough ports for your mixer. I scored a 50' 12/4 (12 send, 4 return) XLR snake for $50.00 on Craigslist. Keep an eye out there.
#3. Option A. Power amp, passive loudspeakers, and sufficient speaker cables.
#3. Option B. My preferred option, active loudspeakers. Keep your eyes peeled, for used. I got my pair of Alto Professional TS115As from a friend, with the associated musiciansgear XLR cables for a deep discount.
#4. HIGH quality surge supressor. A local band to me this past weekend suffered total loss of their PA, damage to tubes in the guitar amp, and a burnt digital mixer when a transformer blew at the bar they were gigging at. probably $6,000.00 worth of damage to their equipment. The equipment protected by surge supressors was fine, everything else let out the magic smoke. LOTS of bars tend to have very bad power / electrical systems, protect your investment!
#5. I already mentioned this, but Mics. Don't let the high dollar is only way to get usable crowd fool you, there are some very good vocal mics for not a lot of money. Nobody is going to compliment your equipment if they know you paid say $20.00 for a Nady mic instead of $100.00 for a Shure, but with vocals of certain ranges, my Nady SP-5s sound better than the Shure SM-58s. Not bashing the Shure, just saying a mic is a pretty personal thing. Every member that will be singing, should provide their mic, mic cable, and stand. Along with pants, underwear, and socks unless you are gigging a nudist colony...
Assuming you can't afford a mixer / PA rig, at least get your own cables, mics, and stands, and RENT a PA / mixer...