The other issue is whether it is a closed Facebook group or a public Facebook group. If it is a closed group then privacy issues do apply. Public groups are accessible by everyone, so broader social media rules apply.
Having said that, talking to your IRB about retrospective approval is essential. Some IRBs do not grant retrospective approvals full-stop, no exceptions. Not having identifiers or including people who have declared themselves or identified how they answer is also an important distinction as well that needs to be in your ethics application. I know some IRBs if they look at the questions and the fact that it is a survey are usually able to grant retrospective approval despite the lack of consent.
Putting a post and asking permission to do research would definitely be helpful. People can raise your concerns and this can be included in your ethics approval and communication with your IRB. Definitely communicate with the group and be transparent in the type of journals and conferences that you could be publishing in. I find details like that is helpful and encourages further engagement from the community. For some, research is a very new thing and they may never encounter an academic journal before. So having this introduction and being part of the scientific writing process can be quite exciting for the group. In some FB groups that I am in, research legitimizes and validates the difficulties that they are having. It gives them a voice when they previously feel unheard. Also, highlighting and educating about the strict privacy regulation of the journal and the IRB process is a great way to educate your group and the general public about research and scientific journals.
Edit: Not sure how I missed the comment about GoogleForms. GoogleForms is not universally accepted as confidential and adequately de-identifying for research purposes. Sometimes when people fill in the GoogleForm, their Google sign in might be collected which then makes the survey not confidential (make sure this setting is off and that this data has not been collected by accident). It is much harder to disprove data manipulation as the survey can be altered as well after being filled in or manipulated by the researchers without any metadata evidence, all of which makes IRBs and ethic boards very nervous and hesitant to approve research using GoogleForms.
For countries which insists that the server of the data storage be in the same country, GoogleForm does not have this option - Google Support Question