If you know their roll20 name you can click on it to go to their profile. From there you can send them a private message with the join link. The join link is located in two places. The first place is on the detail page on the left side called invite player. You will have a pop up that contains the join link that you can copy. The second place is inside the game itself that is hidden behind a gray cover in the chat tab. You usually see it when you first enter the room. You can copy and paste it also into the pm. You can also have them go to the LFG page if it is active and you can click on the blue button "invite to campaign".
