Conditional Custom Page Content

Is the User Registered?

Simply use the iMEMBER definition in a PHP block.

if (iMEMBER) {
echo 'Registered';
} else {
echo 'Unregistered';
}
?>

Checking Group Membership by Id
Use the checkgroup(id) function.

if (checkgroup(2)) {
echo 'Member of group 2';
} else {
echo 'Not a member of group 2';
}

Checking Group Membership by Name
You need to workout the group's id first then use the checkgroup(id) function again.

$allgroups = getusergroups();
$myGroup = 0;
foreach ($allgroups as $g) {
if ($g[1]=='My Group') {
$myGroup=$g[0];
break;
}
}
if ($myGroup) { // group id was found
$myGroup=checkgroup($myGroup);
}
if (checkgroup($myGroup)) {
echo 'Member of "My Group"';
} else {
echo 'Not a member of "My Group"';
}

Note: this code assumes non-membership if the group's id wasn't found.