What are they and why are they helpful?
The start of your corporate gathering/service can set the scene or tone for what's to come, and how you start it will be influenced by many factors. You know your own people best so the skill will be adapting these initial resources. Use them as a springboard for your own worship expressions and styles to emerge. The ethos/atmosphere should be able to make a stranger feel welcome and give the gathered group a sense of the presence of the living God.
You may wish to begin quite informally with a hearty 'Welcome to … (name of your group)', 'Hello, everyone', or 'Hi, it's good to be together today', or other such similar greetings. But what is really important is that you establish the relationship for meeting together - ie, 'To show that when we come together for worship, we recognise that we are all different but all have something to offer. We are the family of God. Each one of us is an individual but joined as part of that family' (Margaret Withers & Tim Sledge, Creative Communion, BRF, 2008, p. 57).
You may feel that at times it is appropriate to be a little more formal and/or liturgical. It not only establishes mutual prayer for one another and God, it also links you with ancient tradition - eg, 'The Lord be with you – and also with you'. This may be short and sweet, but it is profound in its simplicity as it speaks of mutual prayer for one another and an expectation of God's presence.
Sometimes, people like to use a sentence of Scripture in order to put the first emphasis on God, but this doesn't immediately establish a relationship between worshippers. But a sentence or two from Scripture, which is then followed with a more informal greeting, will work well as it links Scripture to ordinary human life, and worship with the rest of life.
It's important too to remember that 'greetings and welcomes' often convey unspoken messages - eg, a person who welcomes or greets people with a face that could turn milk sour rarely inspires everyone to worship, while on the other hand someone who is extremely OTT (over the top) and is verbally (or even physically!) swinging from the chandeliers may be quite frightening, particularly to newcomers or younger folk in your midst if they are too in 'in yer face'. Also, being cajoled to 'shout/sing louder as I can't hear you' when people have just arrived can establish an unhealthy relationship of power that is best avoided.
Greetings and welcomes do not have to be over wordy. They can be creative corporate acts of coming together. Just a simple sharing of a cup of tea can be a profound act of acceptance, especially if it's someone's first time with you and they are feeling emotionally wrung out. Some of the opening activities are ways of saying that we acknowledge that we are all different, but all have something to offer as part of the family of God. All these are in essence communications of welcome, acceptance and belonging. The only limit is our own imaginations, so allow the creative juices to run. The key thing to remember is to use a greeting/welcome that feels right for your context.
Here are a few examples that are not prescriptive or in any way laid down in stone. It will be important for you to adapt things to suit your own style and context. What may 'dive' in one place could be a massive hit and work really well in another.