Integration is always a tricky issue, since NodeBB is a full app in and of itself.
Point your devs to our read APIs. Every link you see can be prepended with api/ to see the publically accessible API.
e.g. https://community.nodebb.org/api/topic/2010/is-nodebb-for-me/3
Obviously, protected pages will require an active session, but most of the pages are publically accessible anyway