If it's an open team meeting (maybe over lunch or a coffee), it's unlikely to be any real "assessment". The focus will strongly lie on judging your "cultural fit". They'll basically try to answer the question of whether they'd like to work with you. Most likely, the topics will be similar to how the team members talk amongst themselves, with an additional focus on getting to know you (and you them). If technical questions come up, this is most likely in the form of asking you about your previous job or general industry-related chitchat (e.g. discussing the merits of different programming languages, the latest gadgets etc.)
Meeting the team is also your chance to ask about the working environment. Do they have a lot of meetings? Are they doing overtime? Who decides what teams work on? etc. You can also ask about topics related to the area, e.g. Where do they go for lunch? How difficult is it to get a nice apartment? What's the typical commute time? and so on.
However, don't make the mistake of assuming you won't be judged. In the end, each of the participants will be asked to give an opinion of you. In my experience, since this is not a strict interview situation, they'll tend to be more lenient towards a candidate, but if something feels off, they might vote against you. Again, keep in mind that they're looking for someone they'd like to work with.
So if you're saying something that makes you seem incompetent (being unable to answer an easy tech related question), disinterested (e.g. asking questions only focused on the city's party life), or potentially difficult to work with (e.g. boastful, stubborn, unfriendly), this will come up in the follow-up discussion.