You can check if a user in the Sendbird application is currently connected to the Sendbird server. For group channels only, you can check whether each member is currently connected to the server.
SbApplicationUserListQueryParams queryParams = new SbApplicationUserListQueryParams();
queryParams.UserIdsFilter = new List<string>{"Jane"};// Added <string> for generic type
SbApplicationUserListQuery query = SendbirdChat.CreateApplicationUserListQuery(queryParams);
query.LoadNextPage((inUsers, inError)=>{if(inError != null){return;// Handle error.}// inUsers[0] is Jane.if(inUsers.First().ConnectionStatus == SbUserConnectionStatus.Online){// Jane is currently online.// UserConnectionStatus consists of online, offline, and notAvailable.}});