Current best guess at requirements:
MacOS 10.14 or later (definitely required): this is available on most Macs starting with 2012 models, except the lower-end MacBooks, which require 2015 or later models (though MacBook Pro models 2012 or later should be okay). Specific machine requirements for 10.14 are at https://support.apple.com/kb/SP777?locale=en_US
Processor: Intel processor (So far, Macs that meet other requirements don’t seem to be limited by CPU; Unreal support for M1 Macs is not released yet)
8GB DRAM (probably, we need more data on this, and this may increase as more of the game is released)
10GB free disk space (for current Avatar Builder release, full game is going to be much larger)
AMD discrete graphics. At least some more recent Intel integrated graphics GPUs seem to work, but frame rates are very poor unless graphics settings are very low, and the frame rates are still pretty low even with low graphics settings. NVIDIA GPUs are no longer supported by Unreal Engine, and it only sort of worked in our test, we saw lots of graphics artifacts.