会议means all kinds of meetings. 会合means get together 。 For example, i'll join u later. The two armies joined forces and marched on. 会means can or be able to. 不知道准不准确。仅供参考

会：general meeting. 会合：meet and become familiar, you could drink alcohol sometimes. 会議：meet and discuss something, you never can drink alcohol there. In the kindergarten, Children have "お誕生会" (birthday party) Parents have, "保護者会"、"保護者の集まり"、”親睦会" or "保護者会の緊急会合" Teachers/workers have "会議" in the working time , and "親睦会" after work.