Chris, thanks for providing this additional information! I'll update the product suggest card I created to share with my team!
Users can currently select "My User Groups" from their profile dropdown and view their user groups. I can bring up the idea of a "Request to Join" button for user groups, but I don't know if an automatic join feature would go through. I think the idea behind our current design is that some user groups consist only of specially qualified volunteers (such as tax preparers) or employees of a company, and there needs to be a way to prevent a volunteer from joining a group they shouldn't be in. There could be a similar issue with teams, where a team leader adds someone to a group (such as an employee group) they shouldn't technically be in. Having people in the wrong user groups could result in inaccurate records, impact reports, etc.
A "request to join" button might solve the problem of individual volunteers, though it could be a challenge in the case where a team leader wants to add a volunteer to a user group. I will bring this up with my team to see what ideas they have. Of course, if you have further ideas and/or could provide more details about why these features would be helpful, that will help me better articulate the argument for these as enhancements.
One more thing that I just thought of: If we had a "Request to Join" button, the current person responsible for approving that request would be a site manager. That wouldn't always be appropriate, since a site manager might not know if someone belongs to a particular group. We don't currently have a "user group manager" type of designation that would meet this need, so that is another issue to consider. As I mentioned earlier, if you can give me more details on the value of this idea to your site and your community, that will help me persuade our development team to further consider everything that would be involved in this enhancement. Thanks!