Q: Tool to convert attendees of an event into an online community?

Hey there,
for our master studies we will organise a small event and we want to convert the attendees to a platform where we can stay in touch with them. I only know of spectrum.chat but maybe some of you knows a better one (or maybe someone sees a business chance in this idea ;) )

It would be nice if:

• Attendees do not need to sign up to view and write posts.
• Optional sign up with mail address.
• Everyone can create community posts and add comments, but only admins can delete and edit stuff.
• The audience will be 20 -70 years old and we don't know yet who will be our target group. So ideally, it should be very easy to use and easily accessible. Creative analogous ideas are also welcome.
• Community members need to be able to see posts from the whole community.
• Ideally free up to at least 100 members or free for social projects.
• Extra points for open source.

Would be cool the get some ideas from you :) Thanks!