Here's some more background info to expand on Dylan's answer.
Understanding open source developers
There are main 3 avenues open source developers get made:
- They are making a tool for themselves, which ends up being useful to quite a lot of people and thus popular.
- They are making a tool for the benefit of a certain community.
- They are making a product which happens to be open source. "Product" here would be something that can be monetized in one way or another, or otherwise has a pathway to becoming financially sustainable.
In case 3, Nash's answer broadly applies; you can send out some feelers on how to work together with the team and they'll likely understand the value UX research brings quite quickly.
In case 2, it's not so simple. If the community doubles as a target audience, you'll likely have an easier time to get going; if the community is the free software and open source software (FOSS) community itself, you'll have a harder time.
Part of the problem here is that, for better and worse, the FOSS community tends to be anti-consumerist, anti-capitalist, strongly pro-privacy and otherwise incompatible with common assumptions we as product and UX people make. After all, it hardly is normal to spend lots of time doing something for free, and continue doing it for free even as the world's most popular websites start using one's software as the foundation of their success.
For example, for us, majorly improving a software for 99% of the userbase while locking out 1% of it would probably be a no-brainer. In the FOSS community however, caring for the remaining 1% is generally seen as worthwhile, even if that results in a plethora of new options and extends development time 3x.
A major contribution to that is aforementioned pro-privacy stance which makes telemetry frowned upon. So when making a change and 20 users pop up in a forum or mailing list demanding a revert, that might be a representative slice of all users, or it might be literally the only 20 people who have ever used the feature you just have changed. The developers will have no idea, so better make the change optional.
In case 1, things are even more stacked against you. Here, in addition to everything which may happen in case 2, it additionally can happen that any bug, feature request, code contribution or anything other than direct praise is seen as an annoyance. Things get added and improved if the main developer(s) feel like it.
Going in this as a UX researcher is bad enough, going in it as a designer is worse still as you're seen not as a potential contributor, but as someone who is about to put more work on the table, which would be a hassle.