Well I've already seen the ending so will it happen ?
Probably not, and Cyclops is kind of a random choice as well considering that outside of the last few years his connection to the Avengers is at best limited. Beast and Wolverine would be the likely X-men to actually be in Avengers movie if it ever happened. Beast has been an off and on member of the Avengers since the 70s and Marvel is just itching to put Wolverine in everything but the point is moot cause Fox ain't giving them up.
Did I miss part of the end that made it look like they could be involved?
I think its because they were both in the Inifinty Arc which is what the Avengers series looks like its leading into. The OP was most likely unaware of the whole Sony/Fox/Marvel thing
No. X-Men and F4 are owned by Fox, Spider-Man (and his supporting characters) are owned by Sony and everything else is owned by Disney. There are some exceptions apparently - like QS and SW, but you won't see the big names in the same movie any time soon.
Unless one of the companies decides they don't like money anymore and givens up their properties..they will keep them forever.
No. It would be more likely even for the X-Men to show up somehow, given how that's only Fox and MARVEL. But in no way would Fox, MARVEL, and Sony ever come to an agreement. Besides, it's not needed, having Spider-man in the Avengers won't draw out more people to see the movie, people who watch the Spider-man movies likely watched the Avengers as well.