Sponsorship
Probationary members are required to find two sponsors to vouch for their good character before they can get vetted.
To find a sponsor, you'll have to meet people and get to know them. The best times to do that are during the Tuesday Open House, Monthly Clean, and by attending classes.
Related: Visiting Protospace
Once you have found someone to agree to sponsor you, ask them to sign your membership forms. They are normally stored in a cabinet in the welcome room.