Someone has asked me to join their co-op and its in the middle of a regatta, if I move to their co-op will I lose my tasks I completed already? Or will they go to my new co-op or be left with my old? And if i leave that co-op will the friends that i have in the old co-op still be in my friends log in my township?
Yes, as soon as you leave a co-op in the middle of the race you forefeit all your current and remaining tasks. If you were on a task it's gone, and you won't be able to complete the rest of them even if you re-join the same co-op (it will set you as not racing if you re-join).
Your old co-op members will only remain as friends if you added them on Facebook or as in-game friends earlier or after leaving.