According to The Wall Street Journal:
Several top tech companies, including Google parent Alphabet Inc. and Facebook Inc., are preparing to file motions in support of Apple Inc. in its court fight with the Justice Department over unlocking an alleged terrorist's iPhone, according to people familiar with the plans of the companies.
Microsoft president Brad Smith said earlier today his company will be filing its own amicus brief supporting Apple in this case.
Update: According to Re/Code, Twitter is planning to throw its support behind Apple with a legal filing as well. The report also mentions that the coalition of tech supporters may file one amicus brief together, rather than as individuals.