The upcoming Marvel's Guardians of the Galaxy is a part of Marvel Cinematic Universe. Is there an official source saying that it'll have a crossover with current storylines in the MCU?
It looks like it.
There’s an interview with Kevin Feige (president of Marvel Studios), who was talking about the post-credits scene at the end of Thor: The Dark World and how it ties into the wider MCU. This mentions Guardians, but spoiler-phobes should look away, as it mentions a potential plot point:
And there’s the first tease at the end of the movie is obviously Guardians, but it also, with the infinity stone, seems to set up some Phase 3 stuff. Is that safe to say?
Yeah. I mean we have… We are always sort of building towards additional things and what’s fun about that reveal for fans and for fans of comics and fans of movies is I think that’s the first time that we reveal that the Tesseract is an infinity stone, that the Aether is an infinity stone. You may see another infinity stone in Guardians and certainly fans of the comics know what one can do with all of those stones if, god forbid, they get them and with Thanos showing up at the end of The Avengers, he plays a part in Guardians and leads us towards at least one potential future.
I think that’s a pretty good suggestion that these are all part of the same storyline.
There are also several characters in Guardians who’ve been mentioned or seen already in the MCU:
Seen before in Thor: TDW:
Taneleer Tivan, the Collector (Benicio Del Toro) was in the Thor: TDW post-credits scene, and is also on the cast listing for Guardians. He's also appeared in some of the trailers.
Some of the official character descriptions (minor spoilers) also mention a character previously seen in the MCU:
Both Gamora (Zoe Saldana) and Nebula (Karen Gillan) have strong personal connections to Thanos, who was seen in the post-credits scene of Avengers and mentioned by Feige above.
Personally I suspect it might take a few films for the storylines to intersect (so I don’t expect to see the Avengers in Guardians, say), but I feel confident in stating that they are probably part of the same shared storyline and will eventually converge.