Alliances

When you join an alliance, do you automatically interline with the other members? 

No. Alliances are independent from interlining contracts.

So then what's the point of being in an alliance? 

This has been discussed widely in the past. If you are interested in getting more details it's worth to do a search.

Basically it is an image gadget with some additional communication feature like a discussion board and decision tool. Besides, some business politics are driven by alliances, so alliances give a bit of more transparency, who is working with whom.