We're looking at redesigning our mobile app and we are also looking at adding new features. To evaluate some of the ideas, I'll need to recruit users that are familiar with the app and/or users who have used a specific feature a couple of times. I'm not sure how to talk to these users. Can you suggest some ideas on how to recruit the right users?
If you have some kind of touchpoint with these users, reach out to them directly. You can directly email them with a personalized email (not just a mail merge!) to ask them if they are interested in providing feedback.
If others in your organization have such touch points and a good relationship with users, they might be better suited to sending out your message if they are comfortable with doing so. Internal people to reach out to can include program managers, salespeople, account managers, and support personnel. If you go this route, you'll likely have to explain to them what you're doing and what the goal is, so be prepared to discuss what you will (and won't) cover in your research. If you go this route, there is a risk that your participant pool will be too engaged or have invalid expectations about the communication that they will have with you, so you will need to make sure that your communications with anyone who you recruit through this method be clear about what they will be doing when they participate in your research and what they should expect as the outcome of it.
You can post to Twitter or Facebook or other social media to reach potential participants if you don't have a direct touchpoint. This generally works best on an active and branded social media account, both in terms of reaching the right participants and the audience of that social media post seeing your attempt as a valid one to gather feedback. When I post to social media looking for participants for research, I usually have created an online survey to help me screen the participants to ensure that they are as engaged as I need for my study and to gather their contact details. I also explicitly ask for retweets/reposts, which helps extend my reach.
You could put a small message in-app to advertise your need for participants. This requires a lot of buy-in from your organization, and (if you're on iOS) will mean that it takes awhile to get participants because your app will need to be updated in the App Store. You have to be very careful to do this in such a way that isn't obnoxious or annoying to your users. You can do it on your website as well if your existing users are likely to visit your site. I treat this as a last-ditch attempt to get participants if I'm just completely stymied because of the cost and the risk associated with getting it wrong.
Whatever method that you use to reach your participants, you need to give them a good reason to participate. For some users, the opportunity to talk to someone who creates the application that they use frequently is sufficient reason. For others, you'll need to provide some other kind of renumeration. Craft a great message that explains why people should participate in your research. The better you do at crafting this message, the more likely that people are to be engaged and motivated.
There are companies that specialize in providing participants for user research. Tell them what profile you need, how many people you need that meet that profile, and when you need them. They'll tell you whether they think it's feasible and how much it will cost you. If they think it's not feasible, they'll often have ideas to make it more feasible.
If you get a few participants but not enough for your research, ask your existing participants if they have any friends or colleagues who they know who also use your app. When I do this, I usually offer some small additional incentive (the size of it depends on how difficult it is to find the participants; my basic currency for this is a $5 gift card per referral). If you've done a really great job of crafting your message about why people should participate, you might find that your request for participation is being shared without you even asking.