Edit: be direct about the fact that you are trying to help people develop communication skills.
In person heavy cultures only work for some people... like they tend to great for 20 somethings, with no kids, who drink, wan't to hang out at bars after work and can take a few days / a week for a retreat without putting a ton of child care burden on their partner if they have one. But if you have kids, or are serious about something outside or work (training for a sport, non profit or open source work, gardening or other hobby) they can be really hard.
Often people just overlook this bias and it works out okay since most people with outside commitments are well established in their careers and have figured out how to be productive and communicate even with people very different from them.
One of the great things about remote first companies is that you get a huge diversity of location, interest and life stage in employees. You get people with families, people serious about a sport they live where they can practice it, people running small farms or fixing up cabins in the mountains or living on aboat, people super active in non profits, always traveling etc...
You really need to lean in on this and encourage people to share their interests and interesting stories, bond over common interests when they are there but also develop the skills and empathy to work with people who are really different them. Like at my current job i totally chat with another outdoorsy middle aged guy with kids in a rural mountain state, but i've also developed a really good working relationship with a young woman in another country, mentor a young guy in a city etc and do regular virtual coffee catch ups with a number of ex coworkers some of whom i've never physically met.
This can be hardest to make work for new grads as for them often it is a choice between moving to a city where they don't know people or living at home...a lot of them do rely on work social networks as they get established. Many remote first companies only hire proven people. Game time and happy hours are ok but hard to make work across time zones.
The think i have seen work well is really focusing on communication skills as an official part of career development. Organize short public speaking / lightning talk / story telling / ted talk / moth radio hour style sessions focused on work and not work stuff. Share a story from your life, teach use something, share your current work in 2 minutes.
Rotating coffee one on ones, mentoring programs and paper reading groups can also work really well.
And like, just directly give people guidance on how to communicate better or let them know that is what you are trying to do.