I am trying to add tags to company using REST API but couldn’t find any API related to it.
Currently there isn’t a way to do this with REST. We have it in our charter to develop this as a part of our V2 roll out for REST. Meanwhile you can try using the grpAssign method and use the company id in place of the contact id (companies are specialized contact records but using the company id works for apply tags with the xml-rpc. But its xml-rpc