If the problem is still occurs, try this PowerShell script:
Add-PSSnapin "Microsoft.SharePoint.PowerShell"
$site = Get-SPWeb -Identity http://<Your site address>
$listname = "Members"
$list = $site.Lists[$listname]
$users = Get-SPUser -Web $site -Group "<The name of the required group on $site>" -Limit all
foreach ($user in $users){
$newItem = $list.Items.Add()
$newItem["Member"] = $user.ID
$newItem.Update()
}
$list.Update()
This script goes through the group and adds each person in the community site.
After the completion all you need to do is ask someone to join the community to recount properly the counter in the "What's happening" web-part.