I have a very big girth and the first time we did it missionary position where she had one leg flat and one knee drawn up high. That was very easy and accomodating for my girth. The second woman tried it with her on top of me but that hurt her at first, but given she was controlling penetration she paused until it was good for her. The third woman we did it her lying on her stomach and me above, and that worked well with my girth too.
The best lube is natural, and all three times we started with vaginal intercourse and used the woman's natural lube and not much extra. Vaginal intercourse also got them more sexually aroused which helped for anal penetration later.