友達 is kind of an odd case - it's a word in the process of fossilisation. 友 on its own is a valid word, albeit one with a distinctly archaic flavour. -たち was then added to make a collective plural (as Thomas Gross says, not a true 'more than one' plural, but instead a 'group described by this term' plural). Modern speakers, though, would always use 友達 in all cases where 友 would have been used in the past (excepting intentional archaisms), meaning that 友達 now also has a singular meaning - the -たち part has fused onto the end of the word and is no longer considered separable. Most people probably wouldn't even think of this -たち here as the collective plural -たち.
'Built-in' is not a term I would use to describe this, though. It's not that 友達 was created as a single noun with -たち already a part, it's instead that the phrase 「友だち」 was so much more common than 友 on its own that people started hearing 友達 as a single unit rather than 友+たち.
However, it's not completely fossilised for all speakers, as some will still mark ?友達たち as sounding odd. (Others are quite fine with it - it's totally fossilised for them.) How do you go about making it plural, then? It's quite simple - the plural of 友達 is 友達. You don't have to do anything with it for it to sound quite natural. If you -really- feel the need to emphasise that you're going with your friends as a group rather than just one, I suppose you can take your chances and hope your listener is fine with 友達たち, but you can easily get away with not bothering - 友達と一緒に行く is quite valid for both 'go with a friend' and 'go with my friends (in general)' (though not for 'go with (some) friends' or 'go with (a couple of) friends'!), and you can always clear up any confusion later.