MicroMenu should be able to open new guild & communities window
In the MicroMenu code line 82 the MicroMenu uses
GuildFrame_Toggle() to open the old guild frame.
Blizzard has included the guild frame into the new Guilds & Communities feature, opened with
ToggleCommunitiesFrame() I didn't test if the LFG feature is supported as well.
Anyhow under most circumstances either the Guild button (or another button) should be available to open the Communities Frame.