Not at all
The only thing to note:
1. It is not illegal to put up close or long range mikrotik / ubiquiti links. Just use the free unlicensed spectrums.
2. It is not illegal to share an internet connection over a network.
3. It can only backfire if you start asking money and it gets around that you're a WISP, then someone has the right to come investigating. Otherwise there will be no proof, and so no warrant will be given to inspect your premises
So just tell your friends to keep quiet about the internet.
1 thing to note about sharing an internet connection
Uncapped:
Uncapped connections are a lot slower than capped connections. Uncapped is usually prepared against downloaders by applying content shaping. Your games will be a LOT faster if you used a capped unshaped sollution such as from Afrihost (R15 per gig)
Gaming:
If you all want to play games online, it might become slightly problematic.
You are all sharing one pipe for your data, so whenever one transmits, the other will have to wait.
This will effect your ping in games. If one of the users decides to start ripping a download etc during a gaming session you will really start to feel it
Im going to do the same as you, within a couple of weeks time ^^
My intent is also to have multiple people share one 4mbit line.