As per category mentioned in the Issue ID, your query is about applicability of GST and NOT Service Tax.
(1)In this scenario, the services of employees are benefiting other distinct persons. Hence such services of employees will be considered as a ‘supply of service’ by one distinct person to another. Company B is liable to pay GST. This activity is covered under Section 7(1)(c) of CGST Act read with Entry No.2 of Schedule-1.
SCHEDULE-1
2. Supply of goods or services or both between related persons or between distinct persons as specified in section 25, when made in the course or furtherance of business :
The following case law will help in arriving at correct decision. The concept is clearer by the decision of AAAR-Karnataka reported as 2019 (20) GSTL. 763 (App.AAR-GST) Applicant : Columbia Asia Hospital Pvt. Ltd. = 2018 (12) TMI 1604 - APPELLATE AUTHORITY FOR ADVANCE RULING, KARNATAKA