When creating a User Group in the foreman UI, there is not a maximum length indicated for the name.
Instead, it will allow names up to 255 chars long. Any name longer than 255 receives this error after clicking submit:

ActiveRecord::StatementInvalid
PGError: ERROR: value too long for type character varying(255) : INSERT INTO "usergroups" ("created_at", "name", "updated_at") VALUES ($1, $2, $3) RETURNING "id"
app/controllers/usergroups_controller.rb:12:in `create'
app/models/concerns/foreman/thread_session.rb:33:in `clear_thread'
lib/middleware/catch_json_parse_errors.rb:9:in `call'