Unfortunately, there are fraudulent services that offer to index journals for a fee. If you receive an invitation from a service, here are a few questions that you should ask.
A web search will often pull up questions raised by other editors or analysts regarding a particular metric source. Failing that, explore their website. Do they give an office address or a contact for data corrections?
Very few reputable metrics will ask you for a fee in return for indexing (and providing metrics) for your journal. Indexing quality content enhances the value of their database and publicizing the metric on your website will direct traffic back to them. So asking for a fee is a gigantic red flag.
So an indexing service calls you up, and tells you that your journal’s Impacting Factoid is 3.662. Thanks for that. But without knowing the calculation of the metric (or what that means in the context of other indexed journals), it is a useless number. If they do tell you the calculation, does it make sense?
Accurate citation metrics rely on a robust citation database that is carefully curated. Cutting corners here results in inaccurate or duplicate indexing of articles. Never trust a metric if you can't identify (and validate) the underlying dataset.
A citation database can only count citations to and from the papers indexed in the database and it is important to know:
For those journals affiliated with a publishing house, such as Wiley, your first step should be to check with your editorial team. Most publishers will have procedures in place to handle data feeds to the many abstracting and indexing websites, and will have experience identifying fraudulent services.