How do accounts get added to groups?

I feel stupid asking this, but I really don't know.

I've got about 15 sock accounts on FB & every time I log in to look at them it seems they've been added to 7 or 8 new spam groups. Groups that I never scraped or had them join.

How are the owners of these groups adding my accounts?

I'd like to know so I can do the same

I also would like to know