Differentiating them based on color is a good idea. You could have the member icon in the club's color and have the visitor icon greyed out.
But you might want to look at it from the other direction instead. Rather than changing the visitors icon to indicate that they lack certain privileges, change the members icon to indicate that they have extra privileges.
You could do this by using exactly the same icon for both groups, but adding a badge to the members icon. This could be a plus sign, a star, or something else that fits the context of the project you're working on, even a currency symbol to indicate that they're paid members.
The good thing about doing it this way is that it provides a very clear element of positive reinforcement to both groups. Members feel good when they see the badge on their icon, since they're probably paying for the privileges that it indicates.
At the same time, visitors don't feel left out at all since their icon is exactly the same as the members, minus the badge, and they understand that the only difference is that they're not paying for the extra privileges.
I'd recommend using a plus sign, as this is the clearest indicator that members have something extra. Differentiating using colors or other vague techniques can make visitors feel unnecessarily marginalized, but by using a plus sign, most people will instantly grasp the difference.
