The P2PU platform is built on the princicple of creating a space with the basic minimum of tools and features to support peer to peer learning. Study groups are free to pull in additional tools from around the web to support their activities and get things done.

If you use external tools or programs they should be:

• Free of charge for everyone.
• Open source if possible.

Other things to keep in mind:

• Learning aspects: If there are particular learning objectives, or styles that you have for the group - choose tools that support them. 
• Technical aspects: Not everyone can evaluate the technical quality of a tool, but it's worth thinking about stability (is the tool reliable and robust) and scalability (if you have 70 people in your group, will the tool support a group that large?). Most tools have been used by other course organizers before so ask about other peoples' experiences before making a final choice if you are not sure.
• Less is more: Offering too many different tools confuses users. Keep it simple and provide links to tutorials or give users basic introductions to the tools you will use.

See a list of tools that the community uses: http://wiki.p2pu.org/community-toolbox and feel free to add to it!