Whilst not immediately obvious…
It is possible to get a list of all users via the groups API endpoint available to admins.
You’ll have to iterate over the following groups:
trust_level_0
trust_level_1
trust_level_2
trust_level_3
trust_level_4
But they do support both limit
and offset
parameters:
/groups/trust_level_0/members.json?limit=50&offset=50