Users can join the organization by adding their name to the list at Wikipedia:Welcoming committee/members or by adding these UserBoxes to their user page:

This user is part of the Welcoming Committee.	((User wikipedia/Welcoming Committee))
This user is a member of the Welcoming Committee.	((User wikipedia/Welcoming Committee 2))
WC This user is part of the Welcoming Committee.	((User WelCom))