Hey sambo92! T-Mobile offers a service called FamilyWhere that can help you locate other lines on your family plan - it does have a fee associated (currently $9.99 monthly) and the other users will have to give permission on their phones to have their devices be located before the service will work.
If the other user is needing to locate their device because it's lost or stolen, then depending on the type of phone they have, they may be able to use a feature like Find My iPhone, Android Device Manager, or if they've set it up and have premium features, LookOut Security to locate their equipment - but the login information would be unique to them.
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