Sending them personal messages may be a great idea, but we have a forum rule about message spam that can lead to a temporary ban, so my advice is to drop a meaningful comment on the service provider that you want to invite to this discussion, and ask after your meaningful post if they are interested to include their service to your topic, and if they are interested they will respond and that's the time to send them a personal invitation.
Don't send them PMs.
Reply on their service threads, and ask them if they would like to be included in OP's list.
Most people do not want to receive PMs from random people as it is equivalent to junk mail. In fact, "Allow Newbies to send you PMs" is disabled by default for accounts. (OP however is no longer a newbie according to the activity and post requirements, but the rank will take a few days to update.)