Well, I'm probably out of my depth giving advice to a lesbian, but I just got through posting on this topic in a different thread. Some women will tolerate a lot of finger penetration, if they are introduced to it in a proper way. Best not to do it at night, I mean, take a whole afternoon where you just spend the day in bed together. You take turns using the standard "G-Spot" technique, which is that you make sure your partner is very aroused and wet, and you go in with the middle and ring fingers of one hand and push up against the spot behind the pubic bone. If a woman has an active G-Spot you will feel something there. But don't be clinical, when done right it's more like a very intimate form of massage, alternating between the G-Spot, and loving attention paid to the clitoris, labia, etc. There's an art to it.

Overall, the women who respond to this technique are in the minority, but among those who do, the results can be very, very intense. One woman I was with could only stand it for a few minutes before she was moaning and thrusting against my hand. Being a guy has it's advantages, you must keep your fingernails short and smoothly filed down. That's really important.