I'm DM'ing a play-by-post campaign and I'm at a point in the adventure where the group all meets for the first time. They've all had something stolen from them at the same time and saw a creature that took something run into a nearby alley (there's 3 seperate parties within the play group currently, as they've only just started, so there's 3 seperate robbers running into alleys in the marketplace).

Ideally, I want the players to have their PCs all give chase so they end up in the same part of town, all looking for the same thing. However, I don't want to railroad my players into doing this. I know I'm not supposed to take away player agency or narrate their actions for them, but the only way I could think of was to tell them that they start giving chase.

So how would you deal with this situation? And more broadly, how can I stop something like this happening again?

If it matters we're currently playing D&D 5e, but the spirit of the question is system-agnostic since the problem is about getting the party together and not about the game system itself.