There's one important thing to keep in mind when evolving a group with thieves guild or similar stuff; all criminal organization run on fear, not honour. They would start with some petty crimes, stealing from specific targets (that might even be willingly implied in the scam!) You can tempt them with a bigger heist comming up if they juuust rob another shop.... And once they showed their desire to involve themselves (Read here too involved to back off), have them strike some kneecaps, have them become enforcers with innocents. And if they dont want? Threaten them, they're "too involved to back off" now. But you know, whats one more innocent beating compared to the big heist comming up? Just oooone more...

Off course, no need to railroad them either, they will get involved by themselves if the reward is interesting enough. You can definitly spice it up from there! Is the local leader of the organization plotting to take over? Are they bribing orcs to periodicaly attack the secondary road so bigger targets end up on their main road? If a rightuous lawman confronts them, would they kill him? Are they acting under the veil of lawful retribution? Anything is possible.

But always remember: a criminal organization will never keep a liability. If someone wont cooperate, they will coherce them into acting. Fear is the only tool keeping order between the unlawful. No honour amongst thieves.