It’s difficult to get USA accountants to join a new online community. It’s dumb to try, they already have a community. They don't need another one.
You can't compete against big communities going head to head. You need to compete where they can't. You need to focus your efforts. Don’t target the total possible audience you want. Pick one segment, possibly two, and focus on them.
Accountants in Boston might be a good start. As might accountants in a particularly speciality or level of experience. You can cater your content and discussions towards them. You can write solely about then. You can give them an experience that none of the USA-wide communities can offer. You can build their peer group.
You can organize events for them, host awards, interview top people, become the go-to place for that group.
Once you have your stable group in place, you can expand…slowly. Develop a unique culture. Broaden your focus. Add all accountants in the state or in broader fields. There’s no rush. Just keep activity high. Ease your way in.
Don’t rush to target everyone. Fight the pressures of growth in favour of success. Pick your focus and build up pressure within that group. Provide an experience bigger communities can’t compete with.