In Powershell, you'll need to import the active directory module, then use the get-adgroupmember, and then measure-object. For example, to get the number of users belonging to the group "domain users", do the following:
Import-Module activedirecotry
Get-ADGroupMember "domain users" | Measure-Object
When entering the group name after "Get-ADGroupMember", if the name is a single string with no spaces, then no quotes are necessary. If the group name has spaces in it, use the quotes around it.
The output will look something like:
Count : 12345
Average :
Sum :
Maximum :
Minimum :
Property :
Note - importing the active directory module may be redundant if you're already using PowerShell for other AD admin tasks.