The mediator may ask the parties to sign the Agreement to Mediate on the day, but a copy will always be sent in advance for your consideration. It sets out the terms of the mediation, fees, timetable and so forth.
Once we have received confirmation from all parties that they wish to proceed with a particular mediator, and on a given date, the Agreement to Mediate will be formally issued and binding from that moment. Remember that a contract does not have to be signed to be binding: acting in accordance with its terms is sufficient.
Pay on time, which will probably be in advance of the mediation. If any party does not pay, the Mediator may not attend, and you may be landed with the other party's costs. Let the mediator or mediation provider know if you are likely to have trouble meting the due date for payment so that an alternative may be agreed if possible.