I'll try to summarize everything I'm thinking about this because I'm not sure I'll be posting much more.
Anyone who wants to participate in what the House of Justice is promoting can roleplay it if they can't be directly involved in it any other way. It can be a way of praying with actions, and of understanding better what the House of Justice is promoting, in order to better encourage and support people we know who are directly involved in it. It can also be rewarding in itself, and help improve a person's life and service to the Faith.
The message "One Common Faith," commissioned and supervised by the House of Justice for all Baha'is to study, says that the community building that the House of Justice is promoting, along with a deep understanding of the process by which humanity’s spiritual life evolves, is the best way for Baha'is to respond to the urgent need to remedy the disease of sectarian hatreds, and free the religious spirit to exert its healing influence on the social order. Anyone who wants to can participate in that community building by studying what the House of Justice says about it, and if they can't actually be directly involved, they can roleplay it, to better understand it and support it, to pray for it with actions, for the rewards in it, and to improve their lives and their service to others.
Besides being the best response for Baha'is to help remedy the disease of sectarian hatreds, I also see it as the best way to help reduce the damage from disasters and help communities recover from them, and the best way to help improve the lives of all people everywhere, now and into the future. I think that the best way for anyone to see that for themselves is to try it. If they can't be directly involved in it, they can roleplay it, to understand it better, and by understanding it better, they can see for themselves if what I'm saying about it is true or not.
Anyone, Baha'i or not, can practice and promote the kind of community building that the House of Justice is promoting, even if they don't believe what we say about God and Baha'u'llah, and even if they have no interest in becoming members, so being the only Baha'i in a community is no reason not to do it.
According to "One Common Faith," the community building is also what the world needs to see, to see that it is possible for all the people of the world, in all their diversity, to live and work together peacefully and happily.in ways that help bring out the best in people and in society. Seeing the community building that we're doing can help strengthen, encourage and inspire anyone who is trying to help improve the world for all people everywhere, no matter if they believe what we're saying about God and Baha'u'llah or not.
The take up rate has been very low sometimes, some places, among Baha'is. I don't think that membership in the Baha'i Faith has anything to do with the interest a person can have in this community development. In one community where 50% of the people were members, 90% of the children were attending the classes. Also, there might be many people who would have no interest at all in the Baha'i Faith who might agree to say prayers together sometimes, for the community and the people in it.