Will Be Joining or Will Join?

Both are correct and they mean the same thing. I would say the difference is esthetic. I find a very slight difference between them: "will be joining" is about his intention, whereas "will join" is about future events, not about anybody's intention.

Will be joining in a sentence?

Be on the lookout for an announcement about what club I will be joining! I'm one of them, and I will be joining Fathers 4 Justice when they launch here. Tomorrow, I will be joining the People's March for Climate, Justice and Jobs in London to call for unity.

Will join you meaning?

1 to come or bring together; connect. 2 to become a member of (a club, organization, etc.) 3 intr; often foll by: with to become associated or allied.