Opt is the opposite of Join. 
Citizen is the opposite of Outsider. Outsider is related to Foreigner. Foreigner is a type of Stranger. you are likely to find Stranger in/at/on Crowd. 
Opt <--Antonym--> Join 
Citizen <--Antonym--> Outsider <--RelatedTo--> Foreigner --IsA--> Stranger --AtLocation--> Crowd 