There is an unofficial way to try to recruit other members into possibly joining your RP but you need to be careful not to overstep your bounds, least you become a nuisance to others - You can try politely asking around once in other members' profile pages (By posting a VM [Visitor Message]) whether they would like to participate in your RP. Briefly explain what it encompasses and wait for a response.
If you do not receive a response within a week or receive a negative response whether immediate or delayed then thank the given individual for listening to your request and do not contact the that individual again about your RP (You can still contact the given individual for other reasons). Anymore than that and you might be reported to the admin for being a nuisance. If you get a positive response then that's great! I hope that this helps you~!