Whether it is best for you to use a group or a site to share information on MLA Commons depends on your communication goals. If you would like to communicate with colleagues who are interested in a certain field or professional issue, you may want to start or join a group. Groups facilitate interaction between group members, since members can participate in discussions, collaborate on docs, and share files. Participation in groups is limited to MLA members. You can read more about groups here.
While participating in a group is a great way to connect with members on topics of interest, sites are by default open to the public and allow you to engage with a community beyond the MLA membership. Sites can be a helpful platform for crafting a professional presence online, circulating information, reaching a broader audience for your scholarship, and posting longer reflections to both members and nonmembers. Sites are covered in more detail here.