Number of participants?

[Guest Green92]

A client of ours only has 30 employees all winter long and at the start of the plan year. They are in construction and hire many more during the year. How do I calculate the number of employees in order to determine if they have to file a 5500 for their welfare plan(s)? Or, where can I go to find this information. Any help is appreciated, I do not normally advise on 5500's outside of retirements plans. This came up at an investment committee meeting and I just want to help them out. Thanks in advance.