I had a situation recently where I joined a volunteer effort and they said, Great, welcome aboard! We'll invite you to the private Facebook group now! I almost declined - as you know if you read this blog at all, I don't do Facebook. But I really wanted in to this group. And, at the time, ALL group business was being conducted on Facebook. Here is how I proceeded.
1) I did **not** re-activate my old FB account. That would have renewed my "friend" connections and re-opened the incoming channel of nonsense and worse. This was why I'd deactivated it months earlier.
2) I created a new account and friended only the one person I needed to in order to enter the private group. Private groups are set up so that you have to be friended to someone in the group before they can invite you.
3) Once in the group, however, I was able to unfriend that person. I had explained to this person what I was doing and why, so no hard feelings there. And now, I had a FB account with zero friends but access to the group feed and resources. The ideal state in which to have a FB account is with no friends, because all of FB's privacy- and sanity-damaging actions are based on who you connect with.
4) I created a #Slack team for the group and started showing people -- especially people in the hierarchy! -- how much better-suited #Slack is for conducting a virtual team than anything FB offers. I set up a number of channels corresponding to the committees I was working on. People took to it quickly and buzz around #Slack started to grow.
Net result, six months later: the group is off FB, fully on #Slack, and both its old private group and my FB account are shut down, existing only in backups. If there.