I feel the Team page accurately reflects the current situation: Ubuntu MATE doesn’t have a formal team anymore, and much of its survival depends on indirect contributions from MATE (upstream), Debian (to package MATE for Ubuntu), Canonical (supporting flavours), and the wider community (testing, bugs, everything else that makes the system work)
Back when I was contributing 5+ years ago, there was more a "team" presence (with quality assurance, ISO testing, bugs reported & fixed, closer collaboration with MATE upstream) but even then, I think we had bottlenecks in communication with our project leader.
What's left today is the 
Ubuntu base + 
MATE + 
Ubuntu MATE specific configuration & themes.
Just to clarify, @vkareh works with MATE upstream, not Ubuntu MATE (but he did hang around with us back then!). See his recent posts here and here.