Could a familar with Spellcasting and Realistic Forum substitute a Group member without the rest of the Group noticing?
Or at least, behave like a normal team-member for the rest of the world?

Yes. to impersonate someone specific, the spirit will need the Con skill. and the Knowledge skill (said person's life story and backround), which you need to have first.
to behave like a normal human an Etiquette skill would be reasonable aswell.

Hm.. all that can be given to a familiar i think without problems.. What do you think about an Familar that acts exactly like the Magican?

In 3rd edition,
We had a raven Shaman bring in an Ally spirit in the form of his 'weirdo cousin'. He was getting 2 shares of the profits for a bit because of that. Luckily for him no-one in the team knew 20th century film well enough to recognize Eric Draven.

Folks figured he was some sort of drugged out adept. Fast as heck, difficult to kill, and completely clueless on occasion.

If any of the party is awakened, you're going to need a way to give your spirit impersonater Masking, so they can hide their aura.

There are a few ways to do this. The easiest is to initiate yourself, get masking and extended masking, and reason with your GM to let you give the Aura Masking power to your spirit. Aura Masking isn't normally a choice for ally spirits, but duplicates masking and extended masking, which you both have, so it makes sense to be able to use arcana to research it or something. After all, you made your familiar's formulae from scratch, anyway.

You could use the magician-spirit karma transfer ritual in street magic (it takes an hour per karma), give it enough karma, and then have it initiate and grab masking. Its a full Magician, it can do that.

You're -definitely- going to want to spend 2 karma on an alternate form, one that definitely looks like yourself.

Don't forget: All the social skills can be defaulted on. If your own social skills aren't great, a high force spirit can roll about the same number of dice as you.

Could you give us a hint where especially in the book you found this ritual? I seem to have missed that one and cant find it.

Street Magic, Page 107, Feeding The Free Spirit.

I am unsure if you can transfer karma both ways, or between non-spirit entities.

Presumably, however, such a ritual cound either be used by a magician/ally(due to having the custom spirit formula), or by using Arcana to research a variation of the same ritual for use by ally spirits.

If you read the Free Spirits and Karma paragraph and the Feeding the Free Spirit paragraph that you referenced it is obvious that it is a one-way feeding tube to a free spirit. Theoretically two free spirits could transfer karma between them but that would be a little counter-intuitive (although a greater spirit giving a gift to one of his minions or a lesser spirit acting as a courier for its master would make sense). But at the very least one subject must be a free spirit and the karma can only be transfered to the free spirit.

It might be interesting to introduce a houserule whereby you could free your ally spirit. It would have more autonomy, but be an NPC instead of an extention of your will. As a GM, I'd allow it to gain Karma on its own ^_^