I think it's very dependent on the norm in the group concerned. for example, I had a birthday meal in a local restaurant on my 50th and some people brought presents or a card, but the majority didn't, and I didn't expect it, as within that group of people (my friends and work colleagues mixed), if it is a meal you pay for yourself, then nothing else is expected.
I had a big 50th birthday party later that year, and almost everyone brought a present and a card (even though not my actual birthday), including the people who had attended the birthday meal. Because if it's a party thrown by someone, that's what is expected.
If I was attending something from MeetUp, I'd not expect to be required to bring a present. I'd view it as extremely informal, and with the birthday as more of a 'reason/excuse' for everyone to socialise. Bit like my 50th in the restaurant was!
I'd be more annoyed at people not bringing a bottle to the Christmas Party tbh! I throw loads of parties, and whilst everyone knows I provide loads of food and drink, they always come with alcohol to sort of 'replace', what I've bought! I think when someone goes to the effort (and expense) of hosting (party, dinner party etc), bringing a bottle or present is mandatory.