online peer support is often used as part of the 'blend' of e-therapy and so I would be interested in joining this group if appropriate as I am interested in group interactions in digital mental health
If you, as a researcher, joined an online group used for peer support you are basically using the members of the support group as participants in a research project. This is true even if you only want to gain insight and are not intending to publish the results. In any research project it is essential to obtain approval from your institution's Research Ethics Committee, and obtain informed consent from all the participants. It is highly unlikely that all members of an online group would give informed consent, so it is very unlikely that you would be able to join the group.
If you are intending to join the group to obtain information from others for research purposes rather than to get peer support for a problem, like others in the group, then you need to take into account the rules governing ethics of research.