You may use groovy script to return list of users applicable for issue assignment, for instance, member of certain group. etc. To do it:

1. Switch to Administration / Groovy Script and add a script to returned desired users. For instance below script returns list of users from dev group:

import io.onedev.server.OneDev
import io.onedev.server.entitymanager.GroupManager

def assignees = []
for (member in OneDev.getInstance(GroupManager.class).find("dev").members) {
	  assignees.add(member.name)
}
return assignees

2. Switch to Administration / Issue Setting / Custom Field and configure the Assignee field to specify Available Choices as Evaluate script to get choices and choose script created in step 1.