I was also overloaded with work and had many doubts about joining the group at all.

overloaded 是动词 overload 的过去分词形式。overload 由前缀 over-（表示"超过"）和动词 load（装载、负担）组成，字面意思是"装载过多"，引申为"使负担过重、超负荷"。 在句中 overloaded 表示被动状态，指"被工作压得喘不过气"的状态。