If you have not reached your desired number of participants and the event is approaching shortly, you should first ask yourself if it makes sense to run the event with less people. If you believe so, problem solved.
If you believe you need to get more people to sign-up, one recommended approach is to reach out to existing participants and ask them for 1-2 names of people they believe would be a good fit for the Lab. Thereafter, ask them to give you warm introductions over email, or even phone numbers so you can call and refer to them as a source.
If you have more time, you can experiment with other ways of filling the spots, such as using online communities and networks, or local people that you know in your city. It tends to be easier to convince someone in-person than to do it online, especially if they have never joined a similar event before.