My group buffer becomes a bit crowded, is there a way to sort my groups into categories so I can easier browse through them?
Gnus offers the topic mode, it allows you to sort your groups in, well, topics, e.g., all groups dealing with Linux under the topic linux, all dealing with music under the topic music and all dealing with scottish music under the topic scottish which is a subtopic of music.
To enter topic mode, just hit t while in Group buffer. Now you can use ‘T n’ to create a topic at point and ‘T m’ to move a group to a specific topic. For more commands see the manual or the menu. You might want to include the %P specifier at the beginning of your gnus-group-line-format variable to have the groups nicely indented.