Ok, while I was driving, I got this alert that said (Gamertag) wants to join my career. Press X to accept or something. What does this mean?
It means you have a career right? And someone wants to join it. To allow them to do so, you have to press “X”. I’d have thought that message was pretty self explanatory.
Basically in dingle player mode you do the races to progress through your “Career”. Now people can join that career and 4 of you can play together to go through career races and tick them off your respective playthroughs.
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Just watch because it will lose you any progress in your current event - I made the mistake of allowing it once in the 5th lap of a 6 lap Goliath Grind…
It was my mistake I pressed the buttons but just watch for it…
Did same on a 3 Lapper of Goliath near the end of lap 2 or 3!!! Still got the Skill points that I had bagged 'til that point but LOST ALL the Credits/XP!
Don’t join in the middle of a long Goliath race. I ONLY get these requests when I’m on Goliath, so I have to turn them down. Otherwise it kicks you out of the race and you have to start over. 
I think I tried it once with someone else and the guy was slamming into every wall, car, you name it, trying to get first. It got annoying so I quit and haven’t tried it again.