Splits are active after every collaborator accepts their invitation to join the Split. Once a Split is active, it will not wait for Split collaborators to connect their bank account in order to run. If you are in a Split but have not connected a bank account, you will not be paid.
You can also tell whether a Split is active by clicking on the individual Split in the 'Splits' tab. An active Split will say "Split is active" in green text under the name of the Split.
