First: As it currently stands, the $User variable does not have a .Users property. In your code, $User simply represents one line (the "current" line in the foreach loop) from the text file.
$getmembership = Get-ADUser $User -Properties MemberOf | Select -ExpandProperty memberof
Secondly, I do not believe you can query an entire forest with one command. You will have to break it down into smaller chunks:
Get-ADUser for each domain (you may have to specify alternate credentials via the -Credential parameterThirdly, to get a list of groups that a user is a member of:
$User = Get-ADUser -Identity trevor -Properties *;
$GroupMembership = ($user.memberof | % { (Get-ADGroup $_).Name; }) -join ';';
# Result:
Orchestrator Users Group;ConfigMgr Administrators;Service Manager Admins;Domain Admins;Schema Admins
Fourthly: To get the final, desired string format, simply add the $User.Name, a semicolon, and the $GroupMembership string together:
$User.SamAccountName + ';' + $GroupMembership;