Let me decompose this to get started, even if that may be just stating the obvious.
- To build a strong community of any kind, you first need to have a community that cares about the matter, and then strengthen it continuously.
- To build a community that cares about the confluence of several things, a good starting point is to engage with each of these existing communities about the missing bits that you care about.
- Strengthening a community requires continuous engagement, including
- documentation of what there is/ was/ ought to be (or not), and changes thereof;
- suitable communication channels.
Now let's apply that to the issue of building and strengthening an open science community in a local setting. This would involve
- engaging with local science communities (researchers, citizen scientists, science journalists, science communicators, science administrators etc., as well as students or researchers who study any of these things) and bringing up issues, opportunities or other things related to open science
- engaging with local open communities (e.g. open city data folks, maker spaces, open-source groups, Wikimedians, OpenStreetMappers etc.) about science
- engaging with the broader open science community about matters that are locally relevant (including in places other than yours, because that might yield inspiration, motivation or notes of caution)
- as well as documenting the above and reaching out about it on channels relevant to your target community.