In the 2015/16 UEFA Champions League competition, two teams from the same domestic league may not be drawn to play in the same group. Furthermore, two teams from the same national association (even if they are in different domestic leagues, eg. the Premier League and the Football League Championship) also may not play in the same group.
From the Regulations of the UEFA Champions League 2015/16 (pdf), p. 22:
Article 16 Match system – group stage
16.01 Once the play-offs have been completed, the 32 remaining clubs are drawn into eight groups of four in accordance with Paragraph 13.05. Clubs from the same association cannot be drawn into the same group.