I will talk about Islamic terrorism that is not limited to ISIS.
Short answer: They are mostly invited. Recruiters go to mosques and look for people who share the same ideology.
These recruiters often come to countries where Islamic conservatism is the main group in the country. These terrorist groups have some specialized people where they are very good at brainwashing people. And they work in small groups, often splitting up and going to their own destinations to find people with potential in the many mosques in small towns or villages. Most new members are invited by others who are already members of the group and they accept the invitation easily because they share the same ideology.
This is mostly correct, just for addition:
I will talk about Islamic terrorism that is not limited to ISIS.
Short answer: They are mostly invited. Recruiters go to mosques and look for people who share the same ideology.
These recruiters often come to countries where Islamic conservatism is the main group in the country. These terrorist groups have some specialized people where they are very good at brainwashing people. And they work in small groups, often splitting up and going to their own destinations to find people with potential in the many mosques in small towns or villages. Most new members are invited by others who are already members of the group and they accept the invitation easily because they share the same ideology.