The aim of this team is that people interested in upstream PureOS work to Debian can coordinate the effort together. PureOS has the policy upstream first - we try to push all reasonable (and acceptable to Debian) changes to Debian so we PureOS itself does not end up having huge delta (patchset) to maintain against Debian.
Note: for historical context, the formation is going to be done by Purism employees that are Debian Developers and Purism employees that are not currently Debian Developers. This (as everything else) is community project so every person is invited.
If you are Debian Developer inside the team it is your responsibility to forward the changes that you or any other PureOS maintainer wants send to Debian be it via sponsoring the package or be it via advising PureOS maintainer the proper way to do, usually act as a bridge to relevant Debian team inside Debian.
If you are not a Debian Developer but you are PureOS maintainer that wants to see their package/changes in Debian, you contact the relevant PureOS developer that is on the list of PureOS Debian Team (list name below).