I have no experience with that service but we can give it a shot (famous last words). 
Edit:
So, for a first draft of the “Rengo” feature to be implemented:
- Minimum number of participating Rengo-Players: 4.
- Players should only be on the online-go.com site and not in need to visit other websites or use of external programs
- Each player has his own dedicated game board where he is to make a move every 1 out of 4 turns (if it’s 4-player Rengo, 1 in n turns in case n-player Rengo with n != 4 even gets implemented, which would be nice but not required.) He makes a move by either clicking on an intersection of the go board to place a stone, or click on pass or resign, just like for normal non-Rengo games.
- For making their move (play a stone, pass or resign) Players just use the same, usual UI functions as for non-Rengo games on OGS, no need for entering coordinates manually or other helper functions.
- Each player just needs one browser tab to participate in the Rengo game, same as for normal non-Rengo games on OGS.
- No need for account sharing or credential sharing, every player can just participate with his normal OGS account like he would in any normal non-Rengo game too.
Good?
Edit more: Thanks for input, added it. And yes, some of the stuff above seems redundant, but whatever.