I agree with PS. well if it were me I would go with 2 options:
1 - Low risk but high risk. put forward a work night out and invite everybody (works well if its a night in particular she is already free on). Low risk that you aren't putting yourself out there but high risk that she would say no.
2 - the next time you bump into her tell her that you're going for a coffee and that she should join you... be playful and tell here "there could be biscuits"
Edit: that's a coffee at work or the coffee shop across the street and use the biscuits as a metaphor for escalation