Thiemo:
There are a couple of ways to approach this in MODx, but let me start by saying that MODx does not provide the infrastructure for a typical community portal site out of the box, or at least not without some custom scripting to implement or integrate such functionality in MODx. That said, my approaches have been:
[*] Use existing MODx webuser system, creating web user groups to represent teams, though you will most likely need to supplement this with an additional table containing any additional data you need to manage that is not already handled (or handled the way you want) by the MODx user system. You would have to implement application level security, as well as custom management interfaces for the user and group data, if you want to have granular security options on who can edit the team information or manage team membersship, etc. I’ve used this approach on numerous sites and it works wonderfully, even as we continue to modify and extend the MODx identity and security systems. This is an important point, as many changes to these systems are being developed already that could affect other approaches that might involve hacking the MODx core code or unsupported usage of or extensions to the core database tables.
[*] Create (or integrate an existing) identity/user/security system based-on or compatible with PHP, into your MODx site, via custom plugins, snippets, and/or modules.
We will hopefully be compiling a set of add-ons in the near future that can serve as a community portal infrastructure for building sites like you describe without any PHP scripting required, but the reality for now is that’s what it takes, and upcoming changes to the MODx core will most likely take precedence over any official work on these kinds of add-ons. But rest assured, that these kinds of capabilities are being, and will continue to be, designed and developed for MODx.
in fact, i’m only a graphics guy, so i hope the bets arent totally lost for me 
MODX Staff