Quick Background. I was inactive for a while because I was teamless.
Is it legal to stand in with another team?
So the definition of what team you are on is only for drive team members and on a per event basis. You can join whatever team you want to.
You just can’t “stand in” for more than 1 drive team in the same competition.
I haven’t competed yet so that should be fine right?
Yes, this is legal, provided no other rules are violated in the process 
By “competition,” meng specifically means “tournament.” Or “event.” It doesn’t matter if you competed on 2114Z this season (hey Jude), you can still compete on 127X at the next tournament, for example.