If you are holding a memorial service in a house of worship, you’ll need to check with the pastoral staff for guidance. But if you’re not holding the service in a house of worship, I believe you have a lot of flexibility in how you structure the service.
The service can be formal or informal, participants sitting in rows or a circle. One individual can lead the service or it can be participatory, for example, asking everyone in advance to prepare something to say if they would like to participate.
The point of a memorial service is to remember the deceased and it's often done with stories; funny and endearing, that depict their qualities and life. It's one last time for all of you to share in the life and memory of the deceased, whether as a friend or family member.
You can choose prayers or one prayer to say, either by clergy, yourself, or all together. The importance of the service is to have a fitting memorial for your loved one so everyone can share in the memories and have some closure.