I would still be a bit careful about giving out that advice trisatek, because what this is saying is "will I, as the person providing the work, get in trouble" it does not make any statements to if the person who has the visa will get in trouble.
Just because you don't have an employment contract doesn't mean you are not working.
Voluntary activity should not amount to either employment, or job substitution . There is a fundamental difference in the nature of the activity itself and the specific exclusion of work (whether it is paid or unpaid) as defined in the individual passport, should not include voluntary activity where it is clearly undertaken as such.
I think the IND was pretty clear that if you are not assuming a role that would ordinarily be used to employ an employable person in the UK, you are not assuming "paid or unpaid work." The documentation above, since it comes straight from the IND, would serve as rather strong legal evidence that volunteering in that case (meeting with the brownies) is not a breach of the fiance visa entry restrictions. This is
especially the case since Brownies is such a well-established service organization. It is very obvious that these women are not compensated and there is no intention of compensating any woman who attends the meetings in this manner.
Volunteering is not equal to unpaid work, according to the IND.
If you are not ready/willing to take the risk based on the information/clarification the IND gives, don't volunteer until you are married. Simple.
