Technically, but "I hope that you will be able to join us as well" would be better.
He is allowed to join. You use the infinitive form in that case. :)
It's not clear - I think you mean, "Either the students or the teachers can join." Or perhaps you mean, "Both students and teachers can join."
Yes, but "joined" should be "join" in order for the question to be grammatically correct.
Yes, however you could also say "if you WOULD join us".
"Definitely i'll join these "is not correct grammar but "I will definitely join this group" is the correct grammar.
Hannah jammed her thumb, and Kayle almost broke her hip and her elbow.
Your question makes no sense grammatically, please resubmit.
"Definitely i'll join these group too" is not correct grammar. The correct grammar is "Definitely, i will join this group too."
I ate the ketchup, and she drank the motor oil; it was a disaster.
"Can I join you?" is the most correct phrase. "Could I join you?" is also acceptable but may sound slightly more formal.
The correct term is decided to join.
No, because "let's" means "let us", and it does not make sense to say "let us join us." It would be correct to say, "Come on, let's join them." It would also be correct, if you are speaking to someone outside your group, to say, "Come on, join us."