Ok so I've wanted to set up a way of receiving Donations for my group and making it optional ie "not to come off too pushy". Anyway I found a slight work-around to the set up. I will be glad to update with feedback from my members once it has had time to percolate out there a while.

If you want to try an "optional donation" maybe this will work for you.

Goto: Group tools/group settings/money/membership dues (don't be afraid of the language) select "Accept payment by cash...." Enter in the description box "made out to (your name), in-person or sent to (your address)"

Below that under [* How much are your Meetup dues] enter in the text box "suggested donation" or "suggested contribution" or other PC version of gimme money. Then next to currency option (assuming US$) keep or change the $5 option to the lowest donation option you would like members to see ( I entered "1.00" note no $ sign) Next to that change "-when-" to your highest $ amount (here I entered "$10.00" but with the $ sign) followed by "Annually" or you could use "anytime". Next, for [What do your dues cover] I entered "Meetup Annual Fee ($144) incidental club cost & future group activities"
Lastly for Refunds I selected custom policy - and entered "Written request with return stamped envelope, mailed or in person"

I share this because it was something I wanted but was turned off by the words dues and fee. Here I hope you find you can set up a payment system in effect "asking" not demanding money from members.

Hope that helps