When I'm having sex with my gf or fingering her I always touch her asshole and play with it/put pressure on it etc.. She has no problem with it and likes it but I'm not sure how to go about getting her to let me actually stick my finger in. I don't know how she'll react. Does anybody have any advice on how to make this happen?

I have found that a few drinks and lube help a lot. If she moans from the pressure just keep gently working it in. Best of luck!

Lots of lube
Lots of gentle rubbing
Don't rush into it, take your time

you should ask her
when things are getting super hot, nibble gently on her ear while rubbing her taint and whisper "i wanna finger your ass..."!
it should be pretty apparent, but communication's pretty important when you're having sex. it doesn't need to be boring and sterile. talk dirty, be dom if she's submissive, make it fun!

my g/f wearing dressing gown. just had a bath. sat next to me on sofa. i knelt on floor in front of her, opened the gown mmm licked her pussy her freshly washed arse so close ... 'your arse look so sexy, may I lick you?' 'mmm yes please' I have never done that before. wow felt so good licking her. 'will you put your finger in please' she said. that lead to a sexy evening as you can imagine. why not try suggesting a bath together and follow what I did. again, always ask ... that will be a turn-on in itself. good luck

If she likes pressure from your finger, she will probably be open to penetration. You could try asking while playing with her asshole but you will need a little lube or at least saliva. It will proably help is she is turned on if you perform oral sex on her while you finger her ass. Of course, alcohol usually helps too. Good luck, one of my biggest turn ons is fingering my wife's ass while she performs oral on me.

Alcohol = bad idea, and even moreso for full-blown anal sex. While it may loosen inhibitions, it also impairs judgment on her part to let you know if there's not-fun pain. That suggestion goes double for any type of "anal ease" numbing creams. With the area numbed, she wouldn't feel any of that type of pain. Anal is definitely fun, but it's a sensitive area down and in there, and easily damaged. You want both of you to enjoy it so it becomes a regular part of your sexual repertoire.

I often finger my girlfriends ass when I'm giving her a good fucking and she loves it. Makes her cum real hard..

If you are licking her pussy let saliva run down to her bum,keep your fingernails very short and just slip your finger in a little bit more each time. She will let you know when it is enough. Do not put that finger into her pussy it can tun nasty and cause infection