Situation

[Fundillo]

My buddy has AT&T, I'm going to get on his Family Plan, on D-Day could I go to the Apple store (across town) and get added to his plan with no hiccups?

 

[guru_ck]

See this thread for your answer: https://forums.macrumors.com/threads/499924/

 

[stagi]

should be fine, don't know why you couldn't. But we really won't know for sure until we know the exact activation process that will happen.

 

[SFStateStudent]

You are exactly what AT&T and Apple is trying to attract. You should be good to go!