Is there an added bonus for being a Top Mafia member for multiple players?

For a single position, no, you get the same Top Mafia bonus whether 1 person or 1000 people have you as their Top Mafia whatever. It doesn't stack.

You can however get the various different Top Mafia bonuses (you can get the bonus for being someone's Mastermind and concurrently get the bonus for being someone's Wheelman too, for example.)

Because 2 of the Top Mafia slots will always be closed to you since Wheelman/Buttonman/Bagman are based on character type, you can get a maximum of 4 bonuses for being in other people's Top Mafia.

Just for easy reference, here's what you get for being in someone else's Top Mafia:

• Mastermind gains a small chance of gaining 50% more experience when performing jobs.
• Wheelman gains a small chance to perform jobs for no energy.
• Button Man gains a small chance to always win fights they initiate.
• Bodyguard gains a small chance to always win fights when they are attacked.
• Safecracker gains a small chance to gain double the money when fighting or robbing.
• Bagman gains a small chance to gain double the money when performing jobs.