Each spider has a spider tattoo with its member number tattooed somewhere on its body. Shizuku's is on her hip, for example.
As they are thieves, anonymity is a valued good, so I can't imagine them going to a tattoo parlor. At least not without disposing of the entire staff afterward.
Does one of the members take to tattooing? But how does this happen then:
All of the spiders are experienced Nen users. When tattooing Hisoka, surely the artist would've recognized that they were not tattooing real skin, but rather Texture Surprise. Since Texture Surprise was inspired by a chewing gum wrapper, I wonder if that would even work in the first place, since tattooing paper is probably not very fruitful.
My guess is that Chrollo stole someone's nen at some point that allows him to personally place the tattoo on someone's body without actually tattooing them. My understanding of Texture Surprise is that it resembles the texture and feel of the actual substance, not that of a paper, and since his nen type is transmutation, he also has a strong affinity with the 2 neighboring groups which are conjuring and enhancing. Texture Surprise is actually a conjuring ability, he probably conjures a layer of skin-like material over a cloth or maybe just transmutes his aura into a skin-like material over the cloth and combines it with bungee gum to stick it (it is highly likely to be the former one).
He may have fooled the members because he's skilled enough to do it, Machi once said that he's so fast that even her eyes couldn't believe it when he applied Texture Surprise over the scars on his hand after she finished stitching his hand. Chrollo may not have expected anyone to be foolish enough to actually challenge them because of their reputation as a ruthless bandit group, and since the entire group was probably there during his "acceptance" into their group, he may have let his guard down and not noticed the fake skin.
But other than that, it's anime logic since it's important for the plot development to take place.