If you want a client under your membership, you must provide a free service for them that is related to your field.
I have no idea about the project but supposing it is a phone application with a certain amount, you can give the first person a free membership, and he will continue for another month. Moreover, if you make a competition and give the winner a free membership for six months, he will continue if he likes it.
So, it all depends on the type of it. At one of the gyms, they gave people a free trial for the first week with the provision of exercises and benefits, so people renew their membership, which made the gym get the maximum number of memberships. However, there are other gyms with no single person!