If you do a Lodestone search of the FC, they should appear in the search results. There, you can view their company profile and also the list of members. Send one of them a tell or

/sea forename SOMEONESFORENAME

to see if they're online. If they have a house, the address will also be listed in their FC profile, so you can visit their house and click their placard or see if they have a guestbook.