Knockl is a free mobile app that helps you find the professional contacts you are looking for.
Press on Events / Groups in the side menu
Press on the + in the top right corner
Add the event hashtag. Your event organiser provides you with the correct hashtag. Note: the hashtag is case-sensitive!
Press on an event to see the participant list. Participants who are already your contacts show an envelope symbol and you can chat with them.
Press on users to see their profile.
If the user is not a contact yet you will see the knocking hand and the cross.
Swipe right to make a contact request.
Swipe left and nothing happens, the user remains in the participant list of the event and you can make a contact request later.