It's usually pronounced like 'Watash tachi' almost omitting the 'i' in the middle. This is usually used in order to have a swiftness in the conversation, which would not be the same case if you were saying 'watashi tachi'. 'Watashi tachi' kind of slows down the conversation pace. In (extremely) formal situations however, I think it's better if you DO USE 'Watashi tachi' making the middle 'i' more audible. Just in case it comes in handy: This is the same case for '~desu'. It's pronounced as '~des' in almost every situation, but when you are trying to be very formal or when you are using 'Keigo/敬語' (honorifics) it's better that you say '~desU', making the final 'u' sound more clearer. With all that said, whether you say 'watash tachi' or 'des', if you speak more slowly and listen carefully, you will realize that you ARE actually pronouncing the final 'i' and 'u' very slightly, almost inaudible but you are pronouncing it! Hope that was helpful! Please ask if you have any more question regarding my answer! m(^_-)v

we pronounce it as “watashi tachi",「わたしたち」，it means "we",maybe some native persons have different ideas.