Help for 23

Invited guests want to comment on images in one of my photogroups but this is what they say:
"Every time I posted a comment I had to type in the spam filter, which was
rather annoying (You know the garbled text and numbers.)"

I'm using the photogroup for research purposes, so really need to know that those invited can use it with ease, and often. At the moment folk have found a way around it by using fotonotes instead. Not ideal for me.

Is there anyway around the captcha filter coming up each and everytime?