That’s good to hear, OP.
In no way excusing the behaviour, are you aware of any offsetting circumstances such as a bereavement, marital problems, work problems etc, that could have left this person unable to act professionally? Do you usually feel comfortable with them?
If one or both of these answers is positive, I think approaching them as a group is a good idea. Perhaps email is the only way you (all) feel able to state your concerns in a calm and objective manner. That’s fine.
But if you can muster the grace to approach them with some genuine curiosity about what went wrong you might get a better result. By that I mean ideally a genuine apology and certainly an explanation that does not attempt to shift any blame to you. Definitely not any ‘I’m sorry you were offended’ nonsense. If the results are not satisfactory you can think about going to their line manager.
If this is just a worsening of a bad attitude towards you (all) things are different. Again I think you will get a better result in person, but I would recommend approaching this person’s line manager first. Again, calm professionalism is important so if you have concerns about that a cogent email is preferable.
Charities run on their volunteers so I very much hope your concerns will be taken seriously.