Create contact for future postdoc without disappointing PhD supervisor
If your advisor doesn't think it is a good idea then you should trust him (this is part of his job, to advise you, after all).

You're just starting out so get your own projects going before trying to connect with another group. In a year or so if you're still interested you could bring this up to your advisor again. You could always attempt to meet people from the other group casually at a conference or something.

Short answer: Listen to your advisor, especially so early in your career.