As Informal groups are formed on common interest of members [AnalysisProject blog]  without any specific form or structure. As Todd Valentine states, Informal project styles are commonly used on very small projects, and when projects are to be completed by small teams who are familiar with both the work to be done and with each other’s style of working [Clarizen, Todd Valentine].Every project got informal groups as they are voluntarily formed by people as they don’t want to stay in isolation. As these group are based on social aspects and does not have any specific goals and within project these groups maintains cultural values develops solid integrity of group which in-fact builds better communication system within group.  Informal group are capable to solve the formal problems of organization even in off-the-job time [AnalysisProject blog].


Group development phases [Forming, Storming, Norming, Performing, Adjourning] is back bone in building team and there are likely and unlikely concerns to be consider being PM should be aware of resulting conflicts withing team and its groups as PM should focus and redirect team to on high performance state, focus and clarify project objectives, make team members interdependent, convert negative energy to positive especially by supporting in some conflicts that may leads to productive results [PMI, Wilemon, David L].


Group cohesion and productivity in relation to leadership shows cohesion group is far more productive [The University of British Columbia, April 1954] through British Columbia university research the groups that were highly or partially structured were more productive than unstructured groups, and semi-structured groups tends to perform more productive in project work compared to highly structured group. Highly structured group consists of members chosen by every member of group, Semi-structured group consists of members chosen by majority/most of members of group, Unstructured group consists of members who are in group by no choice.

References:

Clarizen. 2019. Formal vs. Informal Project Management: Which is Right for You?. [ONLINE] Available at: https://www.clarizen.com/formal-vs-informal-project-management-which-is-right-for-you/.
Project Management: Role of Informal Groups in Organizations. 2019. [ONLINE] Available at: http://analysisproject.blogspot.com/2012/10/role-of-informal-groups-in-organizations.html.
Key Differences. 2019. Difference Between Formal and Informal Groups (with Comparison Chart) - Key Differences. [ONLINE] Available at: https://keydifferences.com/difference-between-formal-and-informal-groups.html.
European Scientific Journal, ESJ. 2019. European Scientific Journal, ESJ. [ONLINE] Available at: https://eujournal.org/index.php/esj/article/viewFile/6562/6333.
Team building in project management . 2019. [ONLINE] Available at: https://www.pmi.org/learning/library/team-building-development-project-management-5707.
Project-Management.com. 2019. The Five Stages of Project Team Development. [ONLINE] Available at: https://project-management.com/the-five-stages-of-project-team-development/.
The relationship between cohesiveness and productivity in small, leaderless, discussion groups - UBC Library Open Collections . 2019. [ONLINE] Available at: https://open.library.ubc.ca/cIRcle/collections/ubctheses/831/items/1.0107036.