In a recent interview with the 'Guardians of the Galaxy' star (get it? Star?) Chris Pratt, Pratt revealed, in regards to his Marvel movie contract, that he was "tied to doing three more, or five more, Guardians of the Galaxy or whatever it is, you know, two more Guardians plus another couple...". Now, we know that one of these "two more" is 'Guardians of the Galaxy 2' and if that is a success (which it probably will be), that will warrant a sequel. That still leaves a minimum of one more and a maximum of three more, based on his wording. This makes the only real viable option one of the Infinity War films, possibly both.
In terms of the Guardians showing up in Infinity War, everybody wants and expects it, although most news has suggested otherwise (given how James Gunn wants to distinguish the Guardians from the other Marvel films) and this is one of the first positive suggestions towards this ideal, besides comments by Dave Bautista that he wanted Drax in the Avengers films and there being heavy rumours that he was confirmed, although those rumours were never confirmed.
The way things are going, considering the timeframe between now and then, this bit of news is possibly the most we can expect, although there's a good chance that they'll appear in the film, given the fan expectation and desire.
So, what are your thought? Should the Guardians be in Infinity War? Personally I think 'yes', given how much I love them and their role in any story involving Thanos.