Sometimes volunteers spell out what they do, plus their other commitments to make people realise that their role is voluntary and unpaid, particularly with organisations like Scouting/ Guiding where parents seem to think that the subs for one of the cheapest children's activities around is enough to pay for rent, resources and enough to pay the leaders a salary.
Some parents can have unrealistic expectations and need it unequivocally stating that the leaders are unpaid and choose to do it on top of their work and family commitments. The leaders aren't moaning that they are "busy" as a martyr, they are explaining their position. Sometimes parents do need to know that you've got to Cubs 10 hours after leaving the house to drop your own kids off at nursery, spend a full day at work, pick the kids up then loiter in the scout hut carpark for 20 minutes because it wasn't worth heading home and that you and your young children will be out for 12+ hours and if you didn't the activity wouldn't exist. The type of people you do have to spell this out to are invariably those who value the benefits the activity least other than as cheap childcare, and are least reliable or far, far too "busy" themselves to help out when required (yet strangely not as busy as the leaders). The genuinely busy ones tend to be more supportive in their own way and be more forgiving that you can't nip down to the Scout shop because little George has just completed a badge and wants it now when the Scout shop is open weekly for two hours clashing with the meeting and there's no chance before it's open on the second Saturday morning of the month, and all the badges will be bought en-mass for presentation at the end of term along with everyone elses.
I've picked up four voluntary roles since becoming a SAHM. One was from before. One was when my DC's activity was in jeopardy due to a lack of leaders, another is reading in school because that extra read per week can be critical to some children, another because junior parkrun needs a set of people stepping forwards each week to be able to run (literally 
). I do these things because I believe in the benefits to children, sometimes mine, sometimes others. I resisted stepping in to save the PTA because it's not my skills set, but I am happy to support events, just not organise them. Ironicly one of the people stepping forwards is crazy busy, but hey a few meetings and hours each term is a barely noticable difference to what she already does.
I've benefited a lot from people putting in the extra mile for me. I'm paying that on now. Hopefully enough youth will be inspired to pass it on in the future.
I'm at my capacity for my own roles and children's activities. Any more would errode our unstructured time too much. Our weekends are low on structured routines. Our busy slot is weekday evenings. The reality is that if we didn't the DCs won't be living some halcyon childhood of taking themselves down the park. Their friends are busy in childcare. There isn't a playing out culture. Summer in the garden is easier, but in the dark winter months the DCs need sports to burn their energy. One has dyspraxia so developing his coordination is particularly important and finding sports he loves and can do is critical to a child who will struggle as PE becomes increasingly team and ball sports based which is not his strength. This busy phase will change over time.