A solution presented itself sooner than I expected, though it's not the one I preferred. For anyone interested, there are a couple of additional options on this pinvoke page. The managed solution did not work for me, but the sample using DllImport worked. I ended up adjusting the sample to handle arbitrary groups based on mapping an enum to SID strings, and including another DllImport for:
[DllImport("advapi32.dll", SetLastError = true)]
static extern bool ConvertStringSidToSid(
string StringSid,
out IntPtr ptrSid);
The modified (working) function looks something like this:
static public bool AddUserToGroup(string user, UserGroup group)
{
var name = new StringBuilder(512);
var nameSize = (uint)name.Capacity;
var refDomainName = new StringBuilder(512);
var refDomainNameSize = (uint)refDomainName.Capacity;
var sid = new IntPtr();
switch (group)
{
case UserGroup.PerformanceMonitorUsers:
ConvertStringSidToSid("S-1-5-32-558", out sid);
break;
case UserGroup.Administrators:
ConvertStringSidToSid("S-1-5-32-544", out sid);
break;
// Add additional Group/cases here.
}
// Find the user and populate our local variables.
SID_NAME_USE sidType;
if (!LookupAccountSid(null, sid, name, ref nameSize,
refDomainName, ref refDomainNameSize, out sidType))
return false;
LOCALGROUP_MEMBERS_INFO_3 info;
info.Domain = user;
// Add the user to the group.
var val = NetLocalGroupAddMembers(null, name.ToString(), 3, ref info, 1);
// If the user is in the group, success!
return val.Equals(SUCCESS) || val.Equals(ERROR_MEMBER_IN_ALIAS);
}
Hopefully this will be of interest to someone else, and I would still like to know if anyone comes across a working, fully managed solution.