Unless you can watch their matches (see that in Fixtures > Friend's fixtures), then you're on different servers and you can't join them - this is most likely your case since you joined 7 days later.
If you happen to be on the same server though, then probably all of the teams in their league were already managed and not bots. In this case, you'll probably be put in the same league next season, that is if all of you remain at the same level.