If you're a guitar player more familiar with tablature than with classic music notation--or just looking to write some simple scores for your ensemble--then Notion Music's Progression is all you need. The interface is easy to learn and use, an onscreen fretboard is provided for entering notes, and the program includes some good sounds so you can preview your work before handing it to the band. The included IK Multimedia Amplitude 2 Duo guitar amp simulation plugin is capable of recreating some very nice guitar sounds as well.
Just because it's relatively inexpensive doesn't mean the quality of notation suffers--the program uses the same engine as the more powerful Notion 3 and produces very nice-looking, basic scores. Unfortunately, there's no demo available at this time, though there are some helpful videos on YouTube.
Note: This link takes you to the vendor's site, where you can find a list of online retailers carrying the latest version of the software. Note that the older Nalpeiron Security licensing program used by Progression (not Notion 3) won't work on a RAID array such as I use on my test PC. Also, you must manually deactivate the program before moving it to another machine. Uninstalling does not do this. However, the weekday-only tech support is very friendly and helpful in this regard.