If a group is constituted then the constitution should set out how the committee are elected or appointed. the basic considerations to avoid trouble are that there is no majority vote on a committee where the people are connected in some way (e.g. relative), signatories to any bank account are unconnected, any conflict of interests are declared and managed.
In terms of getting the best out of people – a skills audit of the management committee would enable you to build on and utilise existing strengths but also help identify where your weak areas are. I’ve attached an example skills audit. Recruiting, selecting and inducting new committee members/trustees is key – the Charity Commission produce useful tools in this area e.g. RS1 which is available at https://www.gov.uk/government/publications/trustee-recruitment-selection-and-induction-rs1
If groups experience difficulties in finding new committee members or volunteers, they can also contact their local volunteering bureau/centre who may be able to help or advise (usually easily found using google).