my friends haven't been able to join my minecraft worlds.

i made a world about two days ago for me and my friend kamryn to play on. she wasn't able to join, and she kept getting a screen that said "unable to join server". i thought it we could just try later and that it wasn't a big deal. today, my sandra wanted to play on our shared server with our friend angel. it was made a few weeks ago. there's only three people that can access it.  but she got a weird screen. it should be attached.